docs: add missing CREATE SECRET end-to-end examples

[kwannoel]

Context

Slack request (2026-02-20): missing CREATE SECRET docs examples.
Marker: docs-eval-1771558446206

Slack thread: (link not provided in transcript)

Related docs/pages:

• https://github.com/risingwavelabs/risingwave-docs/blob/main/sql/commands/sql-create-secret.mdx
• https://github.com/risingwavelabs/risingwave-docs/blob/main/operate/manage-secrets.mdx

Problem

Current CREATE SECRET docs cover syntax and baseline examples, but they do not provide enough practical end-to-end SQL examples for common operator workflows.

Gaps called out by context search:

• No clear rotation flow example showing ALTER SECRET and what users should do for dependent sources/sinks.
• Limited privilege-management examples for USAGE on secrets (grant/revoke + expected behavior).
• Missing broader connector-oriented examples that demonstrate how secrets are referenced in realistic source/sink SQL.
• Missing advanced lifecycle examples (for example, ownership/lifecycle implications and DROP SECRET variants).

This makes secret management harder to discover and increases trial-and-error for users handling credentials securely in production.

Suggested Fix

Add a focused example section (or mini cookbook) under sql-create-secret.mdx and/or manage-secrets.mdx with copy/paste-ready SQL:

1. Create and use secret in connector DDL

• CREATE SECRET ...
• CREATE SOURCE / CREATE SINK reference patterns

2. Rotate secret safely

• ALTER SECRET ... AS ...
• Document expected behavior for dependent objects and recommended operational steps

3. Privileges and access control

• GRANT USAGE ON SECRET ...
• REVOKE USAGE ON SECRET ...
• Include expected failure mode when USAGE is missing

4. Lifecycle cleanup

• DROP SECRET ... patterns, with caveats around dependencies

Acceptance criteria:

• At least one complete end-to-end flow from create -> use -> rotate -> verify -> cleanup.
• At least one RBAC flow with explicit grant/revoke and failure behavior.
• Examples are consistent with both linked docs pages and connector docs terminology.