Add documentation for the various doc auto-generation jobs

This PR should document the miscellaneous jobs auto-generating documentation pages. Note these jobs are essentially used right before release.

Writing these docs raised the question of the branch naming convention for these auto-generated documentation. We have currently a mixture of

- `BRANCH_NAME/{latest,merge}/autogen`
- `autogen/BRANCH_NAME/{latest, merge}`

Should we unify this behavior right now and fix the PR as we go ? /cc @melissalinkert @joshmoore @hflynn @jburel

--no-rebase