docs: Document database interceptors

[marcelomendoncasoares]

Summary

The Serverpod constructor and the withServerpod test helper accept a databaseInterceptor callback that decides what Session.db returns. Nothing in the docs covered it.

Changes

• Add a "Database interceptors" page at the end of the database section, so it does not interrupt the normal CRUD path.
• Show the callback signature, that it receives the session and the framework database, and that it is called once per session.
• Show returning a custom Database implementation and list what it is useful for: tracing, metrics, tenant scoping, policy checks, and safety guards.
• Note that a policy layer has to cover raw access and transactions to be effective.
• Carry over the API stability warning from the implementation: custom layers depend on serverpod_database, which can change in a breaking way in a minor release.
• Show the same callback in a withServerpod test and add databaseInterceptor to the test helper configuration table.
• Cross-link from the raw access page.

Reference

• feat: Add a databaseInterceptor parameter to Serverpod serverpod#5344
• Allow intercepting database requests on the server serverpod#5337

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