Validate RST docstrings

[flying-sheep]

Before switching to Ruff in #147, we used flake8-rst-docstrings, which hasn’t been implemented in Ruff at the time of writing. Me might want to consider a replacement.

rst-roles = ["class", "func", "ref", "cite:p", "cite:t"]
rst-directives = ["envvar", "exception"]
rst-substitutions = ["version"]
extend-ignore = ["RST307"]