Improve inter-document cross-references

[xcoulon]

In particular:

Refer to xref:document-b.adoc#section-b[Section B] for more information.

should check if document-b.adoc was included or not in document-a.adoc and generate the link accordingly (removing the file or substituting the extension)

<a href="document-b.html#section-b">Section B</a>

see https://docs.asciidoctor.org/asciidoc/latest/macros/inter-document-xref/