Address broken references in PDF manual

Summary

The PDF manual is built without some of the content from the man docs tree. This causes warnings during the Sphinx build and strange text in the typeset PDF.

Exact steps to reproduce

See https://manual.gromacs.org/current/manual-2022.2.pdf, section 3.11.105:

For more information about changed features, please check out the release notes (page ??).

It is hard to notice the missing references in the latex build, but you can see the Sphinx missing label warnings when building (the dependencies of) the manual CMake target.

Possible fixes

We could either

• include the release notes in the PDF manual,
• dynamically define macros to produce either internal or external links, and use these macros at the reference sites, or
• surround the sites of such references in .. only:: directives.