Build: `build.jobs` could have `pre_upload` and `post_upload` jobs

[humitos]

I don't think this is immediately needed, but while writing readthedocs/readthedocs.org#9138 I realized that using post_build may not be enough in some cases. If we call a webhook in the post_build that crawls some pages that we just built but the upload step takes a considerable amount of time, the crawler will fetch the older version of the page while expecting the newer one. So, I'm proposing adding pre_upload and post_upload to avoid this kind of case.

[humitos]

I just realized that this is what our Webhook feature does, and we may not want to promote doing this via a post_upload job.