use readthedocs for docs builds

[tonyfast]

@bollwyvl mentioned a 😞 experience for docs build with the system in place now. #17 (comment) the pain can be seen in https://github.com/deathbeds/nbconvert-a11y/actions/runs/7160638366/job/19495254744

we should just lean on read the docs.

[bollwyvl]

GitHub Pages builds are fine, and strictly better for heavy-duty builds which can leverage cache (looking at you, playwright) as long as they have rules in front of them (e.g. only on push/merge to main) so they don't show up in drive-by PRs, I fixed this typo, why is it covered in ✖️ and sending me email?

on: {push: {branches: [main]}}