Build docs for experimental modules (including cell space)

[EwoutH]

Currently docs aren't build for features in mesa/experimental, particularly the cell_space and devs. It would be really usefull to have those docs build in Read the Docs.

It probably requires modifying docs/conf.py.

[stephenfmann]

I think it should just require adding experimental.md with the relevant sphinx commands to docs/apis and including it as new line entry in api_main.md. I don't think you need to modify docs/conf.py.

Relatively easy to do but testing it locally would require a whole sphinx setup.

[EwoutH]

Thanks! Our docs are build in CI so just opening a PR should show us if that’s the case.