CI: Automatically upload docs on release
This should make the publishing of the docs automatic one creating and pushing a new tag. My understanding is that versioneer should take care of showing the version correctly in the docs.

I'd merge this before the 1.5 release in few hours, and if there is any problem, we do the publishing manually once more, and we fix the action for the next release. But I don't think it should fail.

The pdf is missing, but I think we're happy to remove it (I'll open a separate PR for it).

If I'm not wrong, for patch releases we upload the new docs, but delete the previous docs of that minor (so, for 1.4.4 we would delete 1.4.3 docs), and we create a symlink for the previous release. If that's the case, this would still be useful, but we should add in a follow up the deleting and the symlinks.

CC: @simonjayhawkins @mroeschke @lithomas1 @jorisvandenbossche 

> If I'm not wrong, for patch releases we upload the new docs, but delete the previous docs of that minor (so, for 1.4.4 we would delete 1.4.3 docs), and we create a symlink for the previous release. If that's the case, this would still be useful, but we should add in a follow up the deleting and the symlinks.

No we keep the all the patch release docs. AFAIK the only time we overwrite docs is for the release candidate docs (as they all point to, say, 1.5.0) > No we keep the all the patch release docs.

but we do change the symlink.

so at the moment https://pandas.pydata.org/pandas-docs/version/1.4/ points to https://pandas.pydata.org/pandas-docs/version/1.4.3/, but the 1.4.3 docs will not be removed on the 1.4.4 release. Ah, that makes sense. I was checking the server. We do have a `stable` symlink, and then a symlink for each minor version pointing to the last patch release of that version (e.g. `1.4 -> 1.4.3`). Those are the ones that would be nice to update automatically here too. The release candidates seem to have a normal docs build in the server, as the rest of versions.

What this is missing is the zipped docs for the prod docs.