[Docos] Test documentation-embedded code blocks with `pytest` + Sybil = 😶‍🌫️

[leycec]

This feature request is just a helpful reminder to myself that the @beartype test suite should be validating documentation-embedded code blocks with a pytest integration test backed by Sybil – which appears to be the current crème de la crème of the code block testing scene. Someday, someone will complain while pumping their skinny fists that our code blocks are bad and that we should feel bad. Before that happens, let us begin testing them.

This feature request is currently blocked by #408. Since we're rewriting our entire documentation site from Sphinx + reStructuredText into Mkdocs + Markdown, there's little point in tackling this until we finish that. So it goes, bear fam. Progress inches forward one decade at a time.

Thanks so much to long-time AR QA bearthonista @adamtheturtle for the heads-up about Sybil over at unrelated issue beartype/pytest-beartype#15. Prod us if we did nothing in a decade. 🤗