[Doc] need a doc of the feature: Support encryption in WebSocket Proxy

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### What issue do you find in Pulsar docs?

In the release `2.11`, there is a [feature: Support encryption in WebSocket Proxy](https://github.com/apache/pulsar/pull/16234) that provides a way to set encrypt keys for the internal producers and consumers of WebSocket Proxy, but the doc was missing

### What is your suggestion?

Add the missed doc.

### Any reference?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with the linked feature PR #16234 and locate the existing WebSocket Proxy documentation. Document how encryption keys are configured for the proxy's internal producers and consumers, including the release 2.11 behavior described by the feature. Done means users can find and follow the missing configuration guidance.

Written by the indexing model from the issue text.