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?
• [ ] Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
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• [ ] Does your submission pass the doctests? ($ pytest --doctest-plus src/vector/ or $ nox -s doctests)

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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