Integrate C API documentation into docs

George noted:

Check: https://exhale.readthedocs.io/en/latest/index.html. This package is a sphinx extension which helps with the automation of launching Doxygen and generating documentation into Sphinx. The package depends on [Breathe](https://breathe.readthedocs.io/en/latest/), an extention that maps Doxygen and Sphinx documentation. Both packages can be installed with pip.