pipelines: Scope PostgreSQL support boundaries in KFP database guide

[kaikaila]

Description of Changes

Documents the database backend options for Kubeflow Pipelines, and declares explicitly which PostgreSQL deployment configurations are supported.

KFP can be deployed with PostgreSQL as its database backend, but that support is deliberately scoped to the core deployment path. Without an explicit statement of that boundary, operators have no way to tell which combinations are tested and which are merely untried. This adds that statement to the operator-facing documentation.

New page: Database Configuration (operator-guides/configure-database.md)

• Documents MySQL as the default backend, and the overlays that assume it.
• Adds a support scope section for PostgreSQL: the supported configuration, and a table of the combinations that have no corresponding PostgreSQL overlay and are therefore unsupported. Those combinations are also rejected explicitly by the CI deployment script, so they are not exercised in testing.
• Documents the multi-user PostgreSQL overlay (platform-agnostic-multi-user-postgresql), which was not previously covered.
• Clarifies that pod-to-pod TLS has no PostgreSQL overlay, and that the absence of a multi-user TLS overlay is not specific to PostgreSQL.
• Provides a pre-release deployment path using a wrapper overlay, since the released images do not yet include PostgreSQL support.

Installation guide (operator-guides/installation/_index.md)

• Adds a short PostgreSQL section after the pod-to-pod TLS section, summarising the support scope and linking to the new page.

Related Issues

Related: kubeflow/pipelines#13822

This supersedes #4304, which was closed by the stale bot without receiving a review. That PR could not be reopened because the branch has since been rebased onto current master, which orphaned its recorded head commit.

Note on timing

The PostgreSQL deployment instructions describe the state after the PostgreSQL integration lands on master. Until then, the overlays referenced here are not available at the tags the commands resolve to.

Please hold merging until kubeflow/pipelines#13822 is merged.

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