Docs: split review page apart into user docs and tutorial

Problem to solve

https://docs.gitlab.com/ee/user/project/merge_requests/reviews/ is a mixture of features related to reviews, and a sketchy attempt to explain HOW to do a merge request review. This problem has bothered @phikai and me both for quite some time, and I recently had insight into how to fix it.

I'm a little embarrassed it took me this long to spot the solution, actually.

Proposal

1. Clean out the how-to information from the user docs page, and structure the remaining information a bit more sensibly. !148340 (merged) handles this.
2. Take the how-to information, interview a staff+ engineer on best practices, and write a tutorial about the merge review process. !148910 (merged) handles this.
3. Update the left nav after item 2 merges.
4. Check the handbook etc for other places that need updates.

Who can address the issue

Me.

Other links/references

• Related to Clean up 'reviews' docs, create new 'manage' page (!148340 - merged)
• Related to New tutorial: how to do a merge request review (!148910 - merged)