Add instructions for setting ESMFMKFILE and update links to xESMF docs

[tomvothecoder]

Description

• Closes [Doc]: Add instructions for setting ESMF ESMKFILE env variable in Jupyter-based environments #636
• Closes [Doc]: update xcdat Regridding doc and link to/from xESMF #639

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (b835032) to head (46eeada).

❗ Current head 46eeada differs from pull request most recent head ce817c0. Consider uploading reports for the commit ce817c0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #643   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines         1543      1543           
=========================================
  Hits          1543      1543           

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