CURIEs and `conformsTo`

[m-mohr]

We should ensure to tell people that there might be entries in conformsTo that are actually not URLs.

Currently, all examples use URLs and people (including me) may assume that conformsTo are always URLs.
That's not the case as CURIEs are generally allowed by OGC.

See also the following issue for a potential issue appearing on the horizon:
opengeospatial/ogcapi-features#632