Docs: refactor MR index

Problem to solve

The MR index (https://docs.gitlab.com/ee/user/project/merge_requests/) is quite confusing (and way too long) after having ad-hoc content added directly to it. Let's keep it as an index and move specific docs to their own pages.

Proposal

• Clean it up
• Move specific content elsewhere

Who can address the issue

@marcia

When

To be scheduled. It's not a top priority but the doc is looking bad, so, asap. %"Next 3-4 releases"