Add database replication lag troubleshooting to Geo docs

Merged Ben King requested to merge benjaminking-geo-troubleshooting into master
Latest AI-generated summary Experiment

What does this MR do and why?

From data collected over a long-spanning ticket and correspondence with members of Geo and DB teams, a lot of useful information was shared around investigating potential causes for database replication lag.

This MR updates the Geo docs to include troubleshooting different types of database replication lag, and provides a common reason for each value.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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