Update model-guide.rst

[4er4er4er]

I am revising the modeling guide written by Gleb for MP-based solvers, based on updated categories that I developed for recent talks. I have already made some changes to the develop branch, but has become clear that continuing in that way would leave the guide potentially in a confusing state for a few weeks; thus I am working in a different branch from this point onward.

[glebbelov]

Both commits on model-guide.rst look fine to me.

If possible, let's avoid long-lived branches. In any case, let's merge regularly both ways, earlier = better.

[4er4er4er]

I am going to make some revisions now to the syntax terminology in the update branch (AMPL-MP-Modeling-Guide-Revised). This will temporarily cause some inconsistencies, so I won't make further commits until these revisions are done.