Add GitLab Operator as a new product of the docs site

Implements gitlab-org/cloud-native/gitlab-operator#436 (closed).

This is the first iteration to publish the GitLab Operator docs under the docs site. It basically adds another repository we pull from.

The only caveat is the versioning, since the Operator doesn't follow a stable branches process yet. To overcome this, we use its default branch instead.

TODOs:

• We'll need to also think about the docs hierarchy, because right now it's listed under the Charts.
• Some links will now fail because they use relative paths in the Operator docs (outside of (doc/). We need to make them absolute gitlab-org/cloud-native/gitlab-operator!346 (merged)