Docbook writer: Interpret links without contents as cross-references

[jtojnar]

This is what MyST does:

https://myst-parser.readthedocs.io/en/latest/using/syntax.html#targets-and-cross-referencing

[tarleb]

Thank you for the PR and sorry for the late reply! This somehow got lost.

I think this is a good and very sensible change. There is one minor issue with code style, but otherwise this seems good to go.

[tarleb]

For others looking at this PR, here are the docs for xref.

[jtojnar]

Hmm, turns out there was some specialization so we need helper still. Or maybe we could specify default ToString String?

[tarleb]

Wouldn't that (default ToString) still give a warning? We could use Language TypeApplications and write test @Text, but defining testDB5 or similar seems like the easiest solution.

[jtojnar]

Seems to work now.

[tarleb]

Excellent, thanks.

@jgm I'm wondering if we should hide it behind the xrefs_name extension, but I don't think that's necessary.