Rebases: reunite existing UI info, create new page

Problem to solve

We don't have a single home for a page about rebases in the GitLab UI. As a result, information is scattered.

Amy, to Kai:

(We talked in Slack about how https://docs.gitlab.com/topics/git/git_rebase/ focuses purely on the CLI aspects of performing a rebase. That page is not going to move out of topics/git, nor should it grow in scope to include UI things.)

Kai brought up https://docs.gitlab.com/user/project/repository/signed_commits/web_commits/#rebasing-from-ui, and I noted "That info could be rehomed into a new page focusing on rebases in the GitLab UI."

Kai prefers the page live at doc/user/project/repository/merge_requests/rebase.md. I'm fine with that location. Either in /branches or /merge_requests would have worked, as the concept spans both.

Proposal

Find info. Reunite it. Create a new page. Add it to the left nav.

Other links/references

Edited by Amy Qualls