About G2P coverage page

[mellybelly]

We should have an about page that has content that looks something like this:
http://www.mendelian.org/mendelian-traits-numbers
But instead reflects what we know about orthologs in those species that can inform Mendelian diseases for which the molecular basis is not known.

This should use MONDO to unify the diseases from the sources.
Ideally the page could become interactive - how many mendelian diseases with a nervous system phenotype and no molecular basis? for example.

@cmungall @jmcmurry @kshefchek