recommonmark is being deprecated in favour of MyST-Parser. This change
is welcomed, especially since MyST-Parser has syntax extensions for the
Commonmark standard. Effectively we get to use a language that's powerful
and expressive like ReST, but get the simplicity of Markdown.

The rest of this effort will be using some MyST features.

This reorganization efforts aims to remove as much duplication as possible.
The regeneration step once needed is gone, significantly simplifing our
Sphinx documentation configuration.

Also update `docs/requirements.txt`

prettier doesn't support MyST's syntax extensions which are going to be
used in this reorganization effort so we have to switch formatter.

Unfortanately mdformat's style is different from prettier's so time to
reformat the whole repo too.

We're excluding .github/ISSUE_TEMPLATE because I have no idea whether
its changes are safe, so let's play it safe.

MyST-Parser / sphinx's linkcheck complains otherwise.

They're for contributors of Black anyway. Also added a note in the
summary document warning about the lack of attention the reference has
been dealing with.

- add some more detail about Black's beta status
- add licensing info
- add external links in the main TOC for GitHub, PyPI, and IRC
- prepare main TOC for new structure

Not only was the AUTHORS list quite long, this makes it easy to include
it in the Sphinx docs with just a simple symlink.

Yes the document is orphaned but it is linked to in the landing page
(docs/index.rst).

This mostly was a restructuring commit, there has been a few updates but
not many. The main goal was to split "current style" and "planned
changes to the style that haven't happened yet" to avoid confusion.

This is basically a quick start + even more. This commit is certainly
one of most creatively involved in this effort.

This commit was as much restructuring as new content. Instead of being
in one giant file, usage and configuration documentation can expand
without bloating a single file.

Just a restructuring commit ...

This is a promising area of documentation that could easily grow in the
future, let's prepare for that!

This is also another area that I expect to see significant growth in.
Contributors to Black could definitely do with some more specific docs
that clears up certain parts of our slightly confusing project (it's
only confusing because we're getting big and old!).

I know I said I wanted to do these after landing this but given there's
going to be no time between this being merged and a release getting
pushed, I want these changes to make it in.

- drop the number flag for mdformat - to reduce diffs, see also:
  https://mdformat.readthedocs.io/en/stable/users/style.html#ordered-lists
- the GH issue templates should be safe by mdformat, so get rid of the
  exclude
- clarify our configuration position - i.e. stop claiming we don't have
  many options, instead say we want as little formatting knobs as
  possible
- lots and lots of punctuation, spelling, and grammar corrections (thanks
  Jelle!)
- use RTD as the source for the CHANGELOG too
- visual style cleanups
- add docs about our .gitignore behaviour
- expand GHA Action docs
- clarify we want the PR number in the CHANGELOG entry
- claify Black's behaviour for with statements post Python 3.9
- italicize a bunch of "Black"s

Thank you goes to Jelle, Taneli (hukkinj1 on GH), Felix
(felix-hilden on GH), and Wouter (wbolster on GH) for the feedback!

merge conflicts suck, although these ones weren't too bad.

I consider this important enough to be worthy of a changelog entry :)

Prettier works fine for all of the default MyST syntax so let's not
rock the boat as much. Dropping the mdformat commit was merge-conflict
filled so here's additional commit instead.

Lots of wording improvements by Cooper. Taneli suggested to disable the
enabled by default MyST syntax not supported by Prettier and I agreed.
And Jelle found one more spelling error!