docs: Document sending custom Redis commands

[marcelomendoncasoares]

Summary

RedisController.getConnection() and the RedisCommand type are now exported, letting servers send Redis commands that the cache and server events APIs do not expose. The caching page had no coverage of this extension point.

Changes

• Add a "Sending custom Redis commands" section at the end of the caching page.
• Show reading the controller from the session, requesting a connection, and sending a command.
• Explain that the connection is nullable when Redis is disabled or unreachable, including the development fallback case.
• State the ownership rules: the connection is borrowed, not owned, and must not be closed or replaced.
• Note that custom commands write directly to Redis, so keys need namespacing.
• Replace "makes it easy to" in the page intro, per the style guide.

Reference

• feat: Expose the Redis connection for custom extensions serverpod#5391
• Increase the capabilities of the session object by adding more Redis features. serverpod#5333