Add database replication lag troubleshooting to Geo docs

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

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