Warn about broken replication slots filling pg_xlog

What does this MR do?

Adds documentation to the Geo troubleshooting guide about how to deal with the pg_xlog running out of space due to broken replication slots.

Are there points in the code the reviewer needs to double check?

Please take a look at the new format of the troubleshooting.md file for clarity.

Why was this MR needed?

Unused replication slots on the gitlab database can cause cause large amounts of log data to build up in pg_xlog. Several customers have already been hit by this.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #3617 (closed)