Add Google Groups mailing list to docs

[tomvothecoder]

Description

• Closes Create an xCDAT mailing list and public email address #433
• Update links in README and docs homepage
• Separate project motivation from quick description
• Update code of conduct filename using underscores to match GH format
• Update email in code of conduct using core team google group email

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-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (9023209) 100.00% compared to head (bdc9e96) 100.00%.

❗ Current head bdc9e96 differs from pull request most recent head 15615fa. Consider uploading reports for the commit 15615fa to get more accurate results

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Additional details and impacted files

@@            Coverage Diff            @@
##              main      #452   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines         1308      1308           
=========================================
  Hits          1308      1308           

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