Switch from pkg_resources to importlib.resources

[xylar]

This avoids deprecation warnings like:

/gpfs/fs1/home/ac.xylar/mpas_work/MPAS-Analysis/fix-custom-thermal-forcing/mpas_analysis/shared/html/pages.py:19: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  import pkg_resources

This PR also includes a fix to the docstring for ClimatologyMapCustom, which had a glitchy bulletted list that was causing complaints when building the docs.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran the test suite with this branch and the bug fixes from MPAS-Dev/MPAS-Tools#641. Everything completed successfully. Output is here:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xasay-davis/analysis_testing/chrysalis/fix-pkg-resources/