Investigate using Doxylink to link to an HTML Doxygen build of the C++ API reference compiled alongside the main Sphinx site.

This work would build upon DM-22698, which gave Documenteer the ability to run a Sphinx build. In this scenario, Documenteer would build the Sphinx site, and then embed it as a sub-site (subdirectory) of the Sphinx-generated documentation. Doxylink would let us link between from reStructuredText to Doxygen using a custom role. For example:

:lsstcc:`lsst::afw::table::Schema`

Of course, links from Doxygen to the Sphinx site would not be straightforward (in princple, they could be added manually, though that would be prone to fail if URLs change).