[Bug]: No documentation for new network public key feature

Version of OpenTTD

15.0-pre

Expected result

Nice comprehensive documentation about how to securely set up a server that only allows certain people in.

Actual result

Network passwords were removed in https://github.com/OpenTTD/OpenTTD/pull/12323 & https://github.com/OpenTTD/OpenTTD/pull/12300 et al., and people are going to become confused. We need some documentation that we can point at that explains the new methods (and possibly even why they're better)

Steps to reproduce

1. Host a server in old versions
2. Upgrading hosting to 15.0
3. Become confused
4. Post issue on bug tracker
5. Get flamed