Full example for running MOPAC library from C (or Fortran)

[awvwgk]

The README mentions access to an API, it would be great to have a full example on how to use the API from C and/or Fortran as part of the documentation and referenced from the README.

• https://github.com/openmopac/mopac?tab=readme-ov-file#run-from-library

JOSS review: openjournals/joss-reviews#8025