Rewrite GitLab Flow doc

What does this MR do?

This MR rewrites the GitLab Flow documentation at workflow/gitlab_flow.md.

CE MR: gitlab-org/gitlab-ce!20951

Differences from the CE MR:

• Removed a section about rebase before merging a merge request. This behavior has changed since the document was written, and it's not only an EE feature anymore. This change makes the documents identical between CE and EE.

Related issues

No related issues.

Moving docs to a new location?

This MR does not move any docs.

• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well.