Document process for adding new references keys

[karenmajewicz]

[ewlarson]

Examples: Allmaps and georeferencing specifications, oembed

[ewlarson]

geobtaa/geoblacklight_admin#30

[kgjenkins]

Documentation that lists current keys: https://opengeometadata.org/reference-uris/

Are all of these supported/functional in GBL?

[ewlarson]

Need to add COGs and PMTiles to the docs.