Geo documentations improvement for 8.9
Important changes:
- Added note that we don't support MySQL (I don't know if we should add to the limitations or not)
- Added related files to the TOC
- Added disaster recovery instructions (we don't support this, but there are manual steps if anyone need)
- Added troubleshooting instructions
From recent feedback:
We need to improve the SSH keys part of setup instructions, as it's unclear which ssh key you have to add at geo nodes screen and that you have to pre-authorize the connection once (add to known_hosts), so clones can actually work.
Fixes gitlab-org/gitlab-ee#344