Add more information about making nice galleries using nbsphinx + nbgallery

[lwasser]

NOTE: could be worth revisiting this and adding nbgallery as a front end with nbsphinx - will provide a much nicer front end for peope to use https://nbsphinx.readthedocs.io/en/0.8.11/subdir/gallery.html thanks to @astrojuanlu for the tip!

https://docs.poliastro.space/en/stable/gallery.html

tagging this as help-wanted if someone wants to work on this page once we merge the massive documentation pr that is open.