[Docs]: Please add instructions for nginx proxy manager. #431

I confirm that:

• This issue is about the documentation website, NOT about the Navidrome server

Page URL or path

https://www.navidrome.org/docs/usage/integration/authentication/

What's wrong

As the title suggests. It has taken me hours to figure out how to get externalized authentication running for Navidrome with Authentik and NGinX Proxy Manager, and I am still not sure if I've done it correctly.

Suggested fix

No response