Update docs to link to PG streaming replication docs and clarify

What does this MR do?

Updates the Geo external database page to link to PostgreSQL streaming replicaiton documenation. It also clarifies that we expect the user to set up streaming replication for all non-Omnibus managed installs, not just cloud solutions.

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
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• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

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• 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.

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• Clear typos, like this is a typpo.
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Customer ticket: https://gitlab.zendesk.com/agent/tickets/196739 (internal use)