Docs - check for wrong username on Geo secondary

What does this MR do?

I ran into an interesting problem today when setting up a secondary node on Azure and got this error "Geo node has a database that is writable which is an indication it is not configured for replication with the primary node"

I was sure that the database replication was setup correctly.

It turned out after I checked my gitlab.rb file on my secondary, I was using the wrong user for my postgresql connection details.

Once I changed this to the correct user, and ran sudo gitlab-ctl reconfigure, the error went away.

Adding this to the troubleshooting docs, as it might save someone else one day!

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
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When applicable:

• Update the permissions table.
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• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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  • Add docs-only when the only files changed are under doc/*.

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