.gitlab-ci.yml: use environments for documentation versioning

.gitlab-ci.yml: use environments for documentation versioning

This leverages Environments on GitLab to expose different versions of
Knot Resolver docs. The `docs:build` job builds the documentation and
exposes it via job artifacts. Then `docs:develop` (for branches) and
`docs:release` (for tags) take these artifacts and expose them via an
Environment link (an example of this in action may be seen at
[https://gitlab.nic.cz/ostava/knot-resolver/-/environments]).

There is also an optional, manually runnable `docs:public` job, which,
when run, propagates the documentation to the main GitLab Pages of the
project (e.g. [https://knot.pages.nic.cz/knot-resolver]) - this will
probably be mostly used for the latest release, although this setup
pretty much allows us to swap it for whatever version we like at any
time.