Improve C++ migration documentation and link to it from nrnivmodl

[olupton]

#1597 has now been merged, but some follow-up is still needed to improve the documentation and usability.

• The documentation includes a list of ModelDB accession numbers that have been updated for C++ compatibility. It would be more helpful if this included the MOD file names so that one can more easily navigate to a useful pull request.
  • Should this be grouped by accession number or by MOD file name?
• If compilation fails inside nrnivmodl we should emit an error message with a link to the C++ migration documentation. This was discussed in the NEURON dev meeting and might have helped with https://github.com/neuronsimulator/nrn/issues/1875.
• ...?