Aeropropulsive scenario and docs update

[lamkina]

This PR introduces a small change to the aeropropulsive scenario to make he builder names consistent with nomenclature in recent aeropropulsive literature as well as remove confusion with the OpenMDAO balance component. In addition, I added a brief documentation page for the aeropropulsive scenario.

The nomenclature change converts the balance_builder to the bc_coupling_builder and removes it from the coupling group. This is because the standard practice for aeropropulsive problems is to let the optimizer handle the boundary condition coupling with consistency constraints using IDF architecture.

Ideally, @ArshSaja and @anilyil should take a look at this PR as the other main users of the aeroprop scenario.