Update HEMCO country-mask example on RTD to instruct users to create individual country masks (when using GCHP)

[yantosca]

Your name

Bob Yantosca

Your affiliation

Harvard + GCST

Provide a clear and concise overview of the new feature requested.

This follows upon the discussion in issue geoschem/GCHP#432. HEMCO uses special logic to read country mask information that is not present in the MAPL library (which is used for GCHP I/O). A workaround is to create indivdiual mask files for countries and then to specify them in ExtData.rc and HEMCO_Config.rc.

I am creating this issue as a placeholder to remind us to update the HEMCO documentation in this section:
https://hemco.readthedocs.io/en/stable/hco-ref-guide/more-examples.html#scale-or-zero-emissions-with-a-shapefile-country-mask

Reference(s)

No response

Will you be implementing this feature yourself?

Yes

Additional information

No response