Integrate doxygen docs into Sphinx

[mabruzzo]

This introduces a custom extension (doxybuild) to build the doxygen-docs as a part of sphinx.

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Essentially the doxybuild extension adopts the following procedure:

1. At startup:
   1. it constructs a custom configuration Doxyfile that includes the settings from docs/doxygen/Doxyfile and overwrites a few settings (namely the destination where the files are written and the path to the source-code directory)
   2. it generates the doxygen docs in cached build-directory
2. It uses a sphinx-hook to generate a stub index.rst file at the location where the index.html file generated by the doxygen docs will be stored
   • for our case, the file is generated at Reference/internal-api-ref/index.rst
   • this dummy stub file needs to be present during the build to ensure that sphinx properly generates table-of-contents links (i.e. the links provided in the sidebar) to this file.¹
   • the practice of generating dummy stub files was inspired by the behavior of sphinx.ext.apidoc, an extension that is developed as part of the sphinx package²
3. It uses a sphinx-hook that gets triggered at the very end of the build-process to copy the html files generated by doxygen into the appropriate folder (the html file generated by sphinx using the dummy-stub file is overwritten in the process).

A bunch of logic is also generated to properly cache the doxygen build. In other words, iteratively rebuilding the docs won't retrigger doxygen every single time (if we don't care about this, we can cull the behavior)

Footnotes

1. The only real alternative was to put a dummy url-link in the table-of-contents and then replace that absolute link in all html files at the end of the build ↩

2. sphinx.ext.apidoc is used to autogenerate docs for python modules ↩