[newchem-cpp] Add basic doxygen configuration file

[mabruzzo]

The basic premise here is that we may use doxygen to render Grackle's internal API on the documentation website (in the section dedicated to development).

There are 2 ways we could ultimately use this documentation:

• with the Breathe sphinx extension.
• to render separate webpages that we host on ReadTheDocs and link to (but the theming will look very different)

This PR is just adds a basic configuration file and checks to see if we can run doxygen without any errors.