Doxygen for generated files?

If we want to generate C++ Doxygen documentation from generated header files, we'll need to add some scripting to generate the Doxygenfile or see if we can use an environment variable or command line option or something. Otherwise, we should remove our directive to document `NUM_ELEMENTS` (which is currently misidentified as `mrid::NUM_ELEMENTS` instead of `mrid::detail::NUM_ELEMENTS` in `reference.rst`)

Start with reference.rst and the generated header files, then inspect how the Doxygenfile is produced or configured. Decide whether scripting or an environment/command-line option can document generated files; otherwise remove the NUM_ELEMENTS directive. Done means the documentation no longer misidentifies mrid::detail::NUM_ELEMENTS as mrid::NUM_ELEMENTS.

Written by the indexing model from the issue text.