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

[kolesnikovae]

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).

[SolidSnail727]

Hello, is there currently any documentation on how to implement user authentication in Pyroscope? The current Pyroscope UI seems to run with admin permissions by default, I would like to know if there is a way to run it as a user without access to the admin page.