Add docs about path helpers

[adrianthedev]

I've heard that request before.

Maybe we should create simpler path helpers that don't require the full instantiated resource? Maybe it can work with just a string (Avo::Resources::User) and we constantize it and extract the route key. The route key does not require an initialized resource.

One thing to take into consideration and show a warning about, is that if someone uses the record to generate a link, they might not work properly (Photo and PhotoComment scenario). So they might need to pass the full resource.

Maybe use get_resource_by_model_class from the resource manager?