.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.