Reorganize the framework and organization in the developer's guide

[xylar]

This merge organizes the framework and organization documentation in the developer's guide into individual pages for better readability, easier organization, and to facilitate future contributions.

I have also fixed an issue where the example documentation for how to add new test groups to the API wasn't rendering correctly and was getting added to the list of auto-generated docstrings. It is now rendering as example code (as it should) and not autogenerating any documentation based on docstring.

I also added 2 images missing from #33.

I added a section on planar meshes (including considerations for the number of cells in the y direction because of the staggering of hexagons) to the framework/mesh page.

Checklist

• [x] Developer's Guide has been updated
• [x] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [x] Documentation has been built locally and changes look as expected

[cbegeman]

@xylar Thanks for catching that and adding those images

[xylar]

@cbegeman, I think it makes sense to do a similar reorganization of "Organization", which is ridiculously long as well. But I'll wait to see what you think before I do that as well. And I'll probably do it in another PR to keep each PR from becoming overwhelming.

This would be an opportunity to add something about planar meshes related to #37. And there's also some confusion about Step vs. ModelStep in that part of the documentation that needs to be untangled. And I would like to add a description of a new feature that I'm adding (step depending on other steps directly, rather than through input/output files).

[xylar]

...and I changed my mind again after playing around with reorganizing a bit. It doesn't feel like details about running E3SM components or meshes belong in "Organization" so let's stick with "Framework".

[xylar]

Back to draft mode so I can include some more reorganization, including info on planar meshes.

[xylar]

Thanks @cbegeman!