Add documentation page for typing-extensions

[JelleZijlstra]

Currently, typing-extensions is documented only through the README and CHANGELOG. This is hard to navigate, so I feel it's time for a documentation page dedicated to typing-extensions.

We can host it at typing-extensions.readthedocs.io.

[AlexWaygood]

This is great, and very much needed — thank you!

Some comments, mostly pretty minor:

[AlexWaygood]

This is great, and very much needed — thank you!

Some comments, mostly pretty minor:

[AlexWaygood]

A few more small points:

[AlexWaygood]

Make it slightly clearer that this is a distinct point to the previous paragraph:

Suggested change

	Certain types have incorrect runtime behavior due to limitations of older
	versions of the typing module:
	As well as this, certain types have incorrect runtime behavior due to
	limitations of older versions of the typing module:

[JelleZijlstra]

I actually think we should get rid of this catch-all section and document these limitations only under specific objects. I'll move a version of this text to the ParamSpec docs instead, and get rid of the sentence about Sequence/Reversible which I think is only relevant to 3.5-era changes.

[AlexWaygood]

Yes, that makes sense. Sounds like a plan.

[JelleZijlstra]

I couldn't figure out what's supposedly wrong with Concatenate, so I'm going to add a note to ParamSpec only.

Python 3.8.16 (default, Dec 26 2022, 20:36:56) [Clang 14.0.0 (clang-1400.0.29.202)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing_extensions import Concatenate, get_args, get_origin, ParamSpec
>>> P = ParamSpec("P")
>>> c = Concatenate[int, P]
>>> get_origin(c)
typing_extensions.Concatenate
>>> get_args(c)
(<class 'int'>, ~P)

[AlexWaygood]

I couldn't figure out what's supposedly wrong with Concatenate, so I'm going to add a note to ParamSpec only.

Makes sense, though #110might be worth a mention in the Concatenate entry. I took a brief look at it a while back, and it looks... challenging to backport.

[AlexWaygood]

#48 as well...

[JelleZijlstra]

Good point, added those. I also noticed that the :gh: links to CPython weren't working, so I added a custom Sphinx extension to make them work (possibly overkill?).

[AlexWaygood]

possibly overkill?

Definitely a Choice, but I'll support you in it

[AlexWaygood]

Looks great!

[JelleZijlstra]

Next step: get this deployed. Other followups:

• Migrate changelog to RST and put it on this docs page
• Trim down the README to remove redundancy with this page
• Consider extending the introduction, maybe a section specifically on the versioning scheme and/or backward compatibility