DocBook reader: add support for citerefentry (#7437)

Originally intended for referring to UNIX manual pages, either part of the same DocBook document as refentry element, or external – hence the manvolnum element.
These days, refentry is more general, for example the element documentation pages linked below are each a refentry.

As per the *Processing expectations* section of citerefentry, the element is supposed to be a hyperlink to a refentry (when in the same document) but pandoc does not support refentry tag at the moment so that is moot.

https://tdg.docbook.org/tdg/5.1/citerefentry.html
https://tdg.docbook.org/tdg/5.1/manvolnum.html
https://tdg.docbook.org/tdg/5.1/refentry.html

This roughly corresponds to a `manpage` role in rST syntax, which produces a `Code` AST node with attributes `.interpreted-text role=manpage` but that does not fit DocBook parser.

https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#role-manpage

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