Add documentation

[forsyth2]

Add documentation, according to https://docs.e3sm.org/zppy/_build/html/main/contributing.html#initial-setup-obsolete-for-reference-only

This must be done before officially releasing zppy-interfaces.

[forsyth2]

Either documentation needs to be added, or we need to update https://github.com/E3SM-Project/zppy-interfaces/blob/main/.github/workflows/build_workflow.yml to remove the documentation building step. It fails on main: https://github.com/E3SM-Project/zppy-interfaces/actions/runs/16306902360, https://github.com/E3SM-Project/zppy-interfaces/actions/runs/16012849091