Developer Documentation for physical types and intersphinxing

[nstarman]

"Would be good to remember that we should probably document [#11595] in two places: (1) in the documentation part of the contributing guide (here?) to remind documentation contributions to include physical types when quantities/units are involved, and (2) to guide affiliated package maintainers with how to use xref physical types in their own docs, either through intersphinx or by copy-pasting the relevant part of the docs/conf.py file ..." (@adrn)

[nstarman]

E.g. #11680 (comment)