Add instructions on setting up an external Postgres database

[alan-ghelardi]

Improve the documentation by adding instructions on how users can connect the API server to an external Postgres database.

[avinal]

/assign

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
Stale issues rot after an additional 30d of inactivity and eventually close.
If this issue is safe to close now please do so with /close with a justification.
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/lifecycle stale

Send feedback to tektoncd/plumbing.

[avinal]

/remove-lifecycle stale
On hold as per discussed in the WG meeting.