docs: update patch and GitLab info

Cleanly separate the info about making patches vs. working with GitLab.

The Patch Guidelines page describes patch requirements, formatting, reviewing, etc.

The Contributing with GitLab page describes how to create a forked repo, add the git remotes, update your repo, creating topic branches, creating MRs and merging.

Remove all the old information about directly pushing to the master branch.

Reorder topics under the Developer Topics list.