Add documentation for using the `SlackSignature` verifier from `app_backend` with specific Java requirements

1. There seems to be no documentation about how to verify requests using the Java SDK. The only documentation found is the general info.
2. When trying to verify a Slack incoming request, we need to pass in the full URL-encoded body. However, the Java standard library does not URL-encode the aterix character *, thus resulting in an automatic failure if present in the payload. A notice or warning about how to correctly handle that would save some time.

The page URLs

No pages - to add in a new example or page.

Requirements

1. Indicate that the app_backend module contains a helper to validate incoming Slack requests
2. For Spring use, indicate that receiving a ContentCachingRequestWrapper allows for reading a raw request body, else it will be automatically deserialized.
3. When calling the SlackSignature.Verifier method, indicate that the payload must be url-encoded, including for asterix, and that there may be a need to encode the * character manually (to %2A).

Start with the app_backend module and the SlackSignature.Verifier entry point, then review how Spring uses ContentCachingRequestWrapper for the raw request body. Add a new example or page covering Java request verification, URL-encoding the full payload including manually encoding * as %2A, and the stated Spring behavior.