This PR adds `pre-commit` to facilitate and unify the formatting of
everything in this project (code, test, docs, ci).

Aside from `.pre-commit-config.yml` and `docs/.markdownlint.yaml`, all
the other changes are purely cosmetic, applying markdownlint and fixing
here and there to make it pass.

It also creates a new CI job that'll simply execute `pre-commit` for new
PRs.