Source name could be a hyperlink

[IanMayo]

Some rows in the signature table link to another document, or even a named anchor in another document.

These got converted ok, but the DITA was non-compliant, since the target document was missing.

• [x] modify mock data to include URL for source
• [x] modify mock data to include URL and anchor for source
• [x] produce document containing content that gets linked to
• [x] prevent parser from generating URLs for source names
• [ ] when documents are linked, do these checks on document:
  • [ ] it's not the current document
  • [ ] the document hasn't already been converted to DITA
• [ ] if checks pass, convert target document (waiting for #289)
• [ ] provide link to target document

TODO: @IanMayo to reproduce this page of reference content. Also reproduce page of data linked from that reference page data.

Note: the model for this is class A7, with links to docs of type A8

[IanMayo]

Suitable data generated in #286

[IanMayo]

This is handled with our new strategy.