Docs: GL "Reviewers" docs

Move to https://docs.gitlab.com/ee/user/project/merge_requests/reviewing_and_managing_merge_requests.html
Understand and document the differences between assignee and reviewer:
- Concept: what's the difference between a reviewer and an assignee?
- Behavior: does this feature behave exactly as an assignee?
- Notifications: what kind of notifications do we receive?
  - Same as MR assigned to me?
  - Plus a to-do?
- MR filters: do we have a filter for reviewers? (Btw, I think this would be the greatest advantage for us so we can report exactly the MRs we reviewed, vs what we have today: MRs we authored and MRs assigned to us)
GL development development guidelines:
- How should we use this internally:
  - Should we set ourselves as reviewers once we start the review?
  - Should someone seeking a review set us as reviewers instead?

As a very active reviewer, I'd appreciate the following workflow:

The MR author adds me as assignee, representing a review request.
Once I start the review, I add myself as reviewer. This will never change.
I start my review. If I need to send the MR back to the author, I unassign myself and assign the author.
When addressed, they send the MR back to me by assigning me back.
Once I finish my review, I'll unassign myself or merge. But I'll still be listed as a reviewer.

This way, we can effectively track my workload (ofc, of any user):

I can filter MRs assigned to me, which represents the number of MRs waiting for my review.
I can filter open MRs by "reviewer" to retrieve the list of MRs I'm currently reviewing.
I can filter merged MRs by "reviewer" to retrieve the number of MRs I reviewed.

Talk with Daniel Gruesso to get a better understanding of the feature iterate from there.