Investigate blacken-docs

Since we already enforce black, also enforcing that on the sphinx docs would be nice. https://github.com/adamchainz/blacken-docs

Review the existing Black enforcement and the Sphinx documentation setup, then examine blacken-docs to determine how it could be applied here. Done means the approach is validated and, if feasible, documentation formatting is enforced consistently with the repository's existing Black checks.

Written by the indexing model from the issue text.