[ty] implement rendering of `.. code:: lang` in docstrings (#21665)
## Summary
* Fixes https://github.com/astral-sh/ty/issues/1650
* Part of https://github.com/astral-sh/ty/issues/1610
We now handle:
* `.. warning::` (and friends) by bolding the line and rendering the
block as normal (non-code) text
* `.. code::` (and friends) by treating it the same as `::` (fully
deleted if seen, introduce a code block)
* `.. code:: lang` (and friends) by letting it set the language on the
codefence
* `.. versionchanged:: 1.2.3` (and friends) by rendering it like
`warning` but with the version included and italicized
* `.. dsfsdf-unknown:: (lang)` by assuming it's the same as `.. code::
(lang)`
## Test Plan
Snapshots added/updated. I also deleted a bunch of useless checks on
plaintext rendering. It's important for some edge-case tests but not for
the vast majority of tests.