Index all the XML DTD and XSD schemas

These are common enough in codebases and are best matched as they are often missing licensing information

Some pointers:

- Some may be inside recommendations like here: https://www.w3.org/TR/2001/REC-xmlschema-2-20010502/

- Some may have a direct download links (usually the same as their namespace URLs):
- https://www.w3.org/2009/XMLSchema/XMLSchema.xsd
- https://www.w3.org/2001/datatypes.dtd
- https://www.w3.org/2009/01/xml.xsd
- https://www.w3.org/2001/XMLSchema.dtd

See also:
- https://stackoverflow.com/questions/57789454/download-location-for-all-or-at-least-the-most-common-w3c-default-schemas-and