add RFC 3688 concepts #47
Description
https://tools.ietf.org/html/rfc3688 defines registries for DTD public identifiers, XML namespaces, XML schemas, and RDF schemas. all of these are intended as registries for identifiers, and (in case of URI identifiers) intentionally are designed to use URNs, so that it is clear that they are intended to be identifiers.
all of these concepts probably would make sense to be included in web concepts. the registries of DTD public identifiers and RDF schemas are effectively unused (one respectively no entry), but the other ones have seen more uptake:
- XML namespaces: 207 entries
- XML schemas: 124 entries