* Moves some content into spec, deletes duplicate spec content.
* Adds placeholders for where remaining content will be moved. It will be
  split between the spec and the writing_stubs doc.

Moves the section on what to include in stubs with editorial changes only -
e.g., changing "type stub" to "stub file" to match surrounding terminology.

Moves the style guide with a few updates:
* Editorial changes like tweaking code examples to use recommended style.
* Attribute declarations can use assignments when needed to convey extra
  information - e.g., final attributes and enum members.
* Tweaks a reference to the double-underscore convention for positional-only
  arguments to note that it's historical.

The only substantive change is an update to "Comments" to note that many
formats of error suppression comments are allowed.

Added `x: Final = <literal>` to module-level attributes section.
No other changes.

Replaces outdated info with a link to the enums section of the spec.

Added an example of an inner class to "Classes." Removed mention of
asyncio.coroutine from "Decorators" as it's very deprecated. Added functions
decorated with `@dataclass_transform` to list of decorators that type
checkers should recognize.

Removed mention of individual supported/unsupported features from this
section. Where we state that all typing features from the latest released
version are supported, added a caveat with a link to the typeshed feature
tracker.

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

* Reword some sections to focus on type checker behavior.
* Move or remove some sections that fit better in the writing_stubs guide or
  are redundant.

Cuts redundant sections, moves things to writing_stubs, tries out a new
"these should be supported fully" and "these should be supported to at least
the described minimum" structure.

Moves "Library Interface" into spec, further trims redundant text.

Co-authored-by: Carl Meyer <carl@oddbird.net>

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>