Create documentation page for typing

[A5rocks]

I think there's sufficient nuances in trio's typing to merit a documentation page to point at. Off the top of my head:

• that we don't support .spawn yet
• minimum mypy version (maybe pyright version too?) because eg my currently open RaisesGroup types PR requires 1.14+ (at least that's my guess. I remember it doesn't work on 1.13.2 because of the self-types in init)
• relationship with trio-typing (full replacement other than aforementioned not supporting .spawn)
• how to allow nursery-like objects as a 3rd party (just use a protocol w/ the methods/attributes you want), as an alternative to the (closed) PR to export one

There's probably other stuff too, but mainly it would be useful as a link reply to anyone talking about trio-typing.

... do other people think this is a decent idea? Maybe my bullet points above are exhaustive in which case idk maybe we don't need a docs page.

[TeamSpen210]

Seems useful, maybe also we might want to recommend some options to turn on. In particular Mypy's unused-awaitable error code, since in trio coroutine calls must always be awaited.