[User docs] replace Gallery with Photos

[wiswedel]

The user manual still shows the Gallery app: https://docs.nextcloud.com/server/18/user_manual/files/gallery_app.html

That still needs to be replaced by a Photos documentation.

[skjnldsv]

oh wow :p
@jospoortvliet for the communication ;)

[joshtrichards]

Looks like Gallery was shortly after this removed from the docs fully via #5158 (with backports to NC18).

So this is at least 50% done.

Is Photos to a point where it should be included in the User Manual?