gitlab-ci: push docs to the website (manual CI)

As discussed in the status meeting, this adds a CI job that can automatically push the docs to the knot-resolver.cz repository. Intended to be used only on tags, but for whatever emergencies allowed on any ref and/or MR.

It only uploads the docs to a separate docs/<ref_name> branch, the rest (links, merge to master) needs to be taken care of manually during release (at least for now?).