ci: Run doc on PRs, just don't deploy them

[abhinav]

Issues like b7a9571626f0df2083a237174271b74846e1f28e will be caught before merging if we also attempt to build docs for PRs.

Just don't deploy them until they're merged.

[abhinav]

This change is part of the following stack:

• 345 ◀

_{Change managed by git-spice.}