[DB] sqlc walkthrough docs

[cssbruno]

Document a sqlc walkthrough for the DB addon.

Scope:

• Show how user-owned SQL and generated sqlc packages can be used from normal Go handlers.
• Keep GOWDK out of schema ownership and query generation.

Acceptance criteria:

• Walkthrough includes setup, query generation, handler usage, and test commands.
• Docs clearly separate GOWDK runtime plumbing from user-owned database code.