CI move check incorrect sphinx directives to pre-commit

[MarcoGorelli]

Currently, the check (which uses grep with the return codes inverted) doesn't catch

    .. warning:

, while using pygrep from pre-commit (which is a Python implementation of grep with inverted return codes) does. I think this is because grep doesn't count the newline as a character

Advantages of moving this (and, later, other) invgrep checks to pre-commit are:

• they'll be cross-platform
• they'll be simpler to write/maintain because pre-commit comes with pygrep built-in, so there's no need to manually define something like invgrep
• they'll give devs faster feedback on when they introduce unwanted patterns

[jreback]

thanks @MarcoGorelli these pre-commit hooks are nifty