[Doc]: Fix xesmf APIs not appearing in the API Reference page of the latest documentation

[tomvothecoder]

Describe your documentation update

xcdat.regridder.xesmf.XESMFRegridder and xcdat.regridder.horizontal_xesmf are not appearing the API Reference page on main (latest version).

Root Cause

This issue is caused by a change in the Read The Docs build system not having the required dependencies for specific C modules found in ESMF.

• Related issue: Allow pip install --no-deps to meet the dependencies for Sphinx autodoc readthedocs/readthedocs.org#5512

Solution

We need to mock "ESMF" imports because xesmf depends on ESMpy which depends on ESMF (has C modules). The Read The Docs build system does not have dependencies for building these C modules which causes a Python ImportError
with xesmf.

• Solution: https://docs.readthedocs.io/en/stable/faq.html#why-do-i-get-import-errors-from-libraries-depending-on-c-modules