doc: cleanup of doc/index.md

- Add quick link to API docs
- Add marker for external links, to help offline users
- Add information about using 'cargo doc' and the playground
- Clean up some of the wording
- Update body & header style to match rustdoc defaults

Author: tgross35

src/doc/index.md

@@ -4,6 +4,20 @@
 nav {
     display: none;
 }
+body {
+    font-family: serif;
+}
+h1, h2, h3, h4, h5, h6 {
+    font-family: sans-serif;
+}
+h3 {
+    font-size: 1.35rem;
+}
+h4 {
+    font-size: 1.1rem;
+}
+
+/* Formatting for docs search bar */
 #search-input {
     width: calc(100% - 58px);
 }
@@ -21,53 +35,74 @@ nav {
 #search-but:hover, #search-input:focus {
     border-color: #55a9ff;
 }
-h2 {
-    font-size: 18px;
+
+/* Formatting for external link icon */
+svg.external-link {
+  display: inline-block;
+  position: relative;
+  vertical-align: super;
+  width: 0.7rem;
+  height: 0.7rem;
+  padding-left: 2px;
+  top: 3px;
 }
 </style>
 
-Welcome to an overview of the documentation provided by the [Rust project].
-All of these projects are managed by the Docs Team; there are other
-unofficial documentation resources as well!
+Welcome to an overview of the documentation provided by the [Rust
+project]. This page contains links to various helpful references,
+most of which are available offline (if opened with `rustup doc`). Many of these
+resources take the form of "books"; we collectively call these "The Rust
+Bookshelf." Some are large, some are small.
 
-Many of these resources take the form of "books"; we collectively call these
-"The Rust Bookshelf." Some are large, some are small.
+All of these books are managed by the Rust Organization, but other unofficial
+documentation resources are included here as well!
 
-# Learn Rust
+If you're just looking for the standard library reference, here it is:
+[Rust API documentation](std/index.html)
 
-If you'd like to learn Rust, this is the spot for you! All of these resources
+
+## Learning Rust
+
+If you'd like to learn Rust, this is the section for you! All of these resources
 assume that you have programmed before, but not in any specific language:
 
-## The Rust Programming Language
+### The Rust Programming Language
 
-Affectionately nicknamed "the book," [The Rust Programming
-Language](book/index.html) will give you an overview of the language from
-first principles. You'll build a few projects along the way, and by the end,
-you'll have a solid grasp of the language.
+Affectionately nicknamed "the book," [The Rust Programming Language](book/index.html)
+will give you an overview of the language from first principles. You'll build a
+few projects along the way, and by the end, you'll have a solid grasp of how to
+use the language.
 
-## Rust By Example
+### Rust By Example
 
 If reading multiple hundreds of pages about a language isn't your style, then
-[Rust By Example](rust-by-example/index.html) has you covered. While the book talks about code with
-a lot of words, RBE shows off a bunch of code, and keeps the talking to a
-minimum. It also includes exercises!
+[Rust By Example](rust-by-example/index.html) has you covered. RBE shows off a
+bunch of code without using a lot of words. It also includes exercises!
+
+### Rustlings
+
+[Rustlings](https://github.com/rust-lang/rustlings) guides you
+through downloading and setting up the Rust toolchain, then provides an
+interactive tool that teaches you how to solve coding challenges in Rust.
+
+### Rust Playground
 
-## Rustlings
+The [Rust Playground](https://play.rust-lang.org) is a great place
+to try out and share small bits of code, or experiment with some of the most
+popular crates.
 
-[Rustlings](https://github.com/rust-lang/rustlings) guides you through downloading and setting up the Rust toolchain,
-and teaches you the basics of reading and writing Rust syntax. It's an
-alternative to Rust by Example that works with your own environment.
 
-# Use Rust
+## Using Rust
 
-Once you've gotten familiar with the language, these resources can help you
-when you're actually using it day-to-day.
+Once you've gotten familiar with the language, these resources can help you put
+it to work.
 
-## The Standard Library
+### The Standard Library
 
-Rust's standard library has [extensive API documentation](std/index.html),
-with explanations of how to use various things, as well as example code for
-accomplishing various tasks.
+Rust's standard library has [extensive API documentation](std/index.html), with
+explanations of how to use various things, as well as example code for
+accomplishing various tasks. Code examples have a "Run" button on hover that
+opens the sample in the playground.
 
 <div>
   <form action="std/index.html" method="get">
@@ -77,76 +112,143 @@ accomplishing various tasks.
   </form>
 </div>
 
-## The Edition Guide
+### Your Personal Documentation
 
-[The Edition Guide](edition-guide/index.html) describes the Rust editions.
+Whenever you are working in a crate, `cargo doc --open` will generate
+documentation for your project _and_ all its dependencies in their correct
+version, and open it in your browser. Add the flag `--document-private-items` to
+also show items not marked `pub`.
 
-## The Rustc Book
+### The Edition Guide
 
-[The Rustc Book](rustc/index.html) describes the Rust compiler, `rustc`.
+[The Edition Guide](edition-guide/index.html) describes the Rust editions and
+their differences.
 
-## The Cargo Book
+### The `rustc` Book
 
-[The Cargo Book](cargo/index.html) is a guide to Cargo, Rust's build tool and dependency manager.
+[The `rustc` Book](rustc/index.html) describes the Rust compiler, `rustc`.
 
-## The Rustdoc Book
+### The Cargo Book
+
+[The Cargo Book](cargo/index.html) is a guide to Cargo, Rust's build tool and
+dependency manager.
+
+### The Rustdoc Book
 
 [The Rustdoc Book](rustdoc/index.html) describes our documentation tool, `rustdoc`.
 
-## The Clippy Book
+### The Clippy Book
 
 [The Clippy Book](clippy/index.html) describes our static analyzer, Clippy.
 
-## Extended Error Listing
+### Extended Error Listing
 
 Many of Rust's errors come with error codes, and you can request extended
-diagnostics from the compiler on those errors. You can also [read them
-here](error_codes/index.html), if you prefer to read them that way.
+diagnostics from the compiler on those errors (with `rustc --explain`). You can
+also read them here if you prefer: [rustc error codes](error_codes/index.html)
+
 
-# Master Rust
+## Mastering Rust
 
 Once you're quite familiar with the language, you may find these advanced
 resources useful.
 
-## The Reference
+### The Reference
 
-[The Reference](reference/index.html) is not a formal spec, but is more detailed and
-comprehensive than the book.
+[The Reference](reference/index.html) is not a formal spec, but is more detailed
+and comprehensive than the book.
 
-## The Style Guide
+### The Style Guide
 
-[The Rust Style Guide](style-guide/index.html) describes the standard formatting of Rust
-code. Most developers use rustfmt to format their code, and rustfmt's default
-formatting matches this style guide.
+[The Rust Style Guide](style-guide/index.html) describes the standard formatting
+of Rust code. Most developers use `cargo fmt` to invoke `rustfmt` and format the
+code automatically (the result matches this style guide).
 
-## The Rustonomicon
+### The Rustonomicon
 
-[The Rustonomicon](nomicon/index.html) is your guidebook to the dark arts of unsafe
-Rust. It's also sometimes called "the 'nomicon."
+[The Rustonomicon](nomicon/index.html) is your guidebook to the dark arts of
+unsafe Rust. It's also sometimes called "the 'nomicon."
 
-## The Unstable Book
+### The Unstable Book
 
-[The Unstable Book](unstable-book/index.html) has documentation for unstable features.
+[The Unstable Book](unstable-book/index.html) has documentation for unstable
+features.
 
-## The `rustc` Contribution Guide
+### The `rustc` Contribution Guide
 
-[The `rustc` Guide](https://rustc-dev-guide.rust-lang.org/) documents how
-the compiler works and how to contribute to it. This is useful if you want to build
-or modify the Rust compiler from source (e.g. to target something non-standard).
+[The `rustc` Guide](https://rustc-dev-guide.rust-lang.org/)
+documents how the compiler works and how to contribute to it. This is useful if
+you want to build or modify the Rust compiler from source (e.g. to target
+something non-standard).
 
-# Specialize Rust
 
-When using Rust in specific domain areas, consider using the following resources tailored to each domain.
+## Specialized Rust
 
-## Embedded Systems
+When using Rust in specific domains, consider using the following resources
+tailored to each area.
 
-When developing for Bare Metal or Embedded Linux systems, you may find these resources maintained by the [Embedded Working Group] useful.
+### Embedded Systems
+
+When developing for Bare Metal or Embedded Linux systems, you may find these
+resources maintained by the [Embedded Working Group] useful.
 
 [Embedded Working Group]: https://github.com/rust-embedded
 
-### The Embedded Rust Book
+#### The Embedded Rust Book
 
-[The Embedded Rust Book] is targeted at developers familiar with embedded development and familiar with Rust, but have not used Rust for embedded development.
+[The Embedded Rust Book] is targeted at developers familiar with embedded
+development and familiar with Rust, but have not used Rust for embedded
+development.
 
 [The Embedded Rust Book]: embedded-book/index.html
 [Rust project]: https://www.rust-lang.org
+
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+  return tmp.host !== window.location.host;
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+    CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0
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+      anchor.innerHTML += svgText;
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+
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