Build Fortran docs with FORD

[znicholls]

FORD docs: https://forddocs.readthedocs.io/en/stable/

Probably we will need to do something like:

1. compile the Fortran docs with Ford, putting the result in docs
2. build the rest of the docs (including the Python API docs) using mkdocs, without overwriting or mucking up the stuff built with FORD

[znicholls]

Closed by #16