docs: add basic docs for sub-classing awkward mixins

[Saransh-cpp]

Description

Kindly take a look at CONTRIBUTING.md.

Please describe the purpose of this pull request. Reference and link to any relevant issues or pull requests.

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Requests for the required change?
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• Does your submission pass tests? ($ pytest or $ nox -s tests)
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Before Merging

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.77%. Comparing base (6ece748) to head (8d9df39).
Report is 26 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #468   +/-   ##
=======================================
  Coverage   86.77%   86.77%           
=======================================
  Files          98       98           
  Lines       11927    11927           
=======================================
  Hits        10350    10350           
  Misses       1577     1577           

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