Add ocean model descriptions to docs

[cbegeman]

Add some description of the supported ocean models (now just MPAS-Ocean) to documentation. The focus here is configuration options that are most relevant to developing test cases.

Checklist

• [X] Developer's Guide has been updated
• [X] Documentation has been built locally and changes look as expected

[cbegeman]

@xylar This is fairly minimal documentation but I figured something was better than nothing. If you have ideas for more to add to the models/mpas-ocean section I'd welcome suggestions.