Hatching "style" class for GSOD

[brunobeltran]

PR Summary

Goes on top of #18544.

Besides the documentation changes, this now exposes the "density" kwarg to the user, and deprecated non-integer values of "density".

WIP. Only here to document partial completion for GSOD.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[jklymak]

@brunobeltran its pretty hard to comment on this with so little info above, and so much extraneous info in the commit. What are we supposed to look at?

Of the top of my head I don't know why the density argument would need to be an integer...

[github-actions]

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