Change legend guide to object oriented approach

[dmatos2012]

PR Summary

Addresses #20754 to change legend guide from all pyplot interface to the ax objected oriented approach

PR Checklist

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

[story645]

Not sure if one of the plt examples should be left in as a resource but otherwise is awesome.

[dmatos2012]

thanks @story645 for the review. I've updated the docs acording to the reviews. However, as per issue #20553, I think it would be nice to have some sort of description of the different loc descriptions for axes. It can be quite tricky to understand at first.

edit: flake8 linting 🤦‍♂️

[story645]

totally agree w/ you about adding in something on location, possibly as an expansion of https://62747-1385122-gh.circle-artifacts.com/0/doc/build/html/tutorials/intermediate/legend_guide.html#legend-location & maybe using the cheatsheet as inspiration?
https://github.com/matplotlib/cheatsheets/blob/master/cheatsheets-2.png

[dmatos2012]

totally agree w/ you about adding in something on location, possibly as an expansion of https://62747-1385122-gh.circle-artifacts.com/0/doc/build/html/tutorials/intermediate/legend_guide.html#legend-location & maybe using the cheatsheet as inspiration?
https://github.com/matplotlib/cheatsheets/blob/master/cheatsheets-2.png

I was thinking perhaps something like creating a 3x3 (or 2 x2 w.e) subplot_mosaic and just placing the legend in the different loc positions, and then link to the cheatsheet.png for their future reference. But given that this is mentioned on a different issue, I was thinking on creating another PR just for that, unless you clearly want it on this one instead. lmk

[story645]

I think it's a good idea to leave it to another issue since that's gonna be a bigger change to the guide content so folks might have more opinions.