Improve contributor guide with respect to API documentation

[sbenthall]

It occurs to me based on a recent exchange that we haven't really laid out a good procedure for reviewing documentation changes on new PRs.

Which might explain why our docs are in the state they are in.

This part of the contribution guide is incomplete, since it doesn't describe how to add new files/modules to the API docs, or test the documentation build:
https://docs.econ-ark.org/guides/contributing.html#documentation-convention

That could be added.