Add documentation on customising logrotate config

[Encephala]

As discussed in #1317, it isn't feasible to add configuration options to the compose file for customising logrotate functionality, as way too many options exist.

However, customising the config isn't very hard to do, so added a section to the docs on the Advanced Configuration page.

[Encephala]

Whoops sorry for the force push, I pushed to the wrong branch

[nginxproxymanagerci]

Docker Image for build 3 is available on DockerHub as jc21/nginx-proxy-manager:github-pr-3422

Note: ensure you backup your NPM instance before testing this PR image! Especially if this PR contains database changes.

[jgkennedy]

./logrotate.custom:/etc/logrotate.d/nginx-proxy/manager should be ./logrotate.custom:/etc/logrotate.d/nginx-proxy-manager (last slash should be a hyphen)

[Encephala]

whoops, you're entirely right!