Add API reference for GauXC

[awvwgk]

• Mention CMake options for enabling C and Fortran API
• Update paths for tarball and reference skala branch
• Add a detailed reference for the CMake options and move CMake details there, reference during installation guide
• Add molecule section (C++, C, Fortran, and HDF5 reference)
• Add basis set section (C++, C, Fortran, HDF5 reference, and shell ordering conventions)
• Add grid section (C++, C, Fortran, and enums)

Closes #31

[stefabat]

Everything looks good to me, with the updated link to download gauxc I have no problem enabling the C and FORTRAN support.