[DOC] Document Pyroscope authentication options and provide an example setup

Now that we don't have the means to manage users and API keys in Pyroscope, we need to provide authentication and hardening guidelines to users. I think we should also have an example of authenticating agent/SDKs via proxy such as nginx (ideally for both single-/multi-tenant deployments).

No files or tests are named in the issue. Start by locating the authentication documentation and existing single- and multi-tenant deployment guidance, then document available authentication and hardening options with nginx proxy examples for agent and SDK access. Done means users can follow the examples for both deployment modes.

Written by the indexing model from the issue text.