improve OMF about page

[danielskatz]

the about page (https://openmodelingfoundation.github.io/about/) says:

"The Open Modeling Foundation is a meta organization"

This is not particularly helpful. I was hoping to find out what modeling means to OMF, what kinds of models are in-scope vs out-of-scope, how OMF is organized (which could be a pointer to the governance page), what organizations or people are part of it, how one joins (also hiding in governance).