Clarify volumes configuration required for web server when used with FPM w/ additional volumes

[joshtrichards]

At a minimum whatever volumes one has placed /var/www/html + /var/www/html/apps + /var/www/html/custom_apps on needs to be available to the web/frontend.

Magically taken care of if not using additional volumes (since it all ends up in the /var/www/html volume), but since many people do use additional volumes to split things up with greater granularity (without fully understanding all the implications) we should probably find ways to clarify things in the README in a couple spots + the examples (embedded comments?).

i.e.
https://github.com/nextcloud/docker/tree/master?tab=readme-ov-file#additional-volumes

e.g. to prevent #1241 (and others that I'm not going to bother searching for but have helped people out with here or on the forum before on a recurring basis).

[joshtrichards]

Related: #1851