Remove "Learn Rust" from TRPL.

Some history:

While getting Rust to 1.0, it was a struggle to keep the book in a
working state. I had always wanted a certain kind of TOC, but couldn't
quite get it there.

At the 11th hour, I wrote up "Rust inside other langauges" and "Dining
Philosophers" in an attempt to get the book in the direction I wanted to
go. They were fine, but not my best work. I wanted to further expand
this section, but it's just never going to end up happening. We're doing
the second draft of the book now, and these sections are basically gone
already.

Here's the issues with these two sections, and removing them just fixes
it all:

// Philosophers

There was always controversy over which ones were chosen, and why. This
is kind of a perpetual bikeshed, but it comes up every once in a while.

The implementation was originally supposed to show off channels, but
never did, due to time constraints. Months later, I still haven't
re-written it to use them.

People get different results and assume that means they're wrong, rather
than the non-determinism inherent in concurrency. Platform differences
aggrivate this, as does the exact amount of sleeping and printing.

// Rust Inside Other Languages

This section is wonderful, and shows off a strength of Rust. However,
it's not clear what qualifies a language to be in this section. And I'm
not sure how tracking a ton of other languages is gonna work, into the
future; we can't test _anything_ in this section, so it's prone to
bitrot.

By removing this section, and making the Guessing Game an initial
tutorial, we will move this version of the book closer to the future
version, and just eliminate all of these questions.

In addition, this also solves the 'split-brained'-ness of having two
paths, which has endlessly confused people in the past.

I'm sad to see these sections go, but I think it's for the best.

Fixes #30471
Fixes #30163
Fixes #30162
Fixes #25488
Fixes #30345
Fixes #28713
Fixes #28915

And probably others. This lengthy list alone is enough to show that
these should have been removed.

RIP.

Author: steveklabnik

src/doc/book/README.md

@@ -14,31 +14,25 @@ Even then, Rust still allows precise control like a low-level language would.
 
 [rust]: https://www.rust-lang.org
 
-“The Rust Programming Language” is split into eight sections. This introduction
+“The Rust Programming Language” is split into sections. This introduction
 is the first. After this:
 
 * [Getting started][gs] - Set up your computer for Rust development.
-* [Learn Rust][lr] - Learn Rust programming through small projects.
-* [Effective Rust][er] - Higher-level concepts for writing excellent Rust code.
+* [Tutorial: Guessing Game][gg] - Learn some Rust with a small project.
 * [Syntax and Semantics][ss] - Each bit of Rust, broken down into small chunks.
+* [Effective Rust][er] - Higher-level concepts for writing excellent Rust code.
 * [Nightly Rust][nr] - Cutting-edge features that aren’t in stable builds yet.
 * [Glossary][gl] - A reference of terms used in the book.
 * [Bibliography][bi] - Background on Rust's influences, papers about Rust.
 
 [gs]: getting-started.html
-[lr]: learn-rust.html
+[gg]: guessing-game.html
 [er]: effective-rust.html
 [ss]: syntax-and-semantics.html
 [nr]: nightly-rust.html
 [gl]: glossary.html
 [bi]: bibliography.html
 
-After reading this introduction, you’ll want to dive into either ‘Learn Rust’ or
-‘Syntax and Semantics’, depending on your preference: ‘Learn Rust’ if you want
-to dive in with a project, or ‘Syntax and Semantics’ if you prefer to start
-small, and learn a single concept thoroughly before moving onto the next.
-Copious cross-linking connects these parts together.
-
 ### Contributing
 
 The source files from which this book is generated can be found on

src/doc/book/SUMMARY.md

@@ -1,10 +1,7 @@
 # Summary
 
 * [Getting Started](getting-started.md)
-* [Learn Rust](learn-rust.md)
-    * [Guessing Game](guessing-game.md)
-    * [Dining Philosophers](dining-philosophers.md)
-    * [Rust Inside Other Languages](rust-inside-other-languages.md)
+* [Tutorial: Guessing Game](guessing-game.md)
 * [Syntax and Semantics](syntax-and-semantics.md)
     * [Variable Bindings](variable-bindings.md)
     * [Functions](functions.md)