Documentation tasks

[davissp14]

• [x] Document quorum requirements.
• [ ] Document how to recover a fenced primary.
• [x] Document how to recover when cluster goes read-only due to capacity.
• [x] Document how to manually unregister a member in the event flyctl fails to unregister it on removal.
• [ ] Document how to view repmgr events via repmgr cluster events.
• [ ] Document Stolon -> Repmgr migration.

Note - All of these docs should probably be included in Fly's core docs.

[guillaumervls]

Some material is in the docs folder