PR Summary

Adds https://mpl-interactions.readthedocs.io/en/latest/ (github here) to the list of 3rd party packages. The main utility provided by the library is controlling the parameters to a function you plot using sliders or other widgets. At first, this was ipympl specific, but after some growing pains, it now works in any backend by using matplotlib sliders if not in a notebook. This made me feel ok renaming it from ipympl-interactions to mpl_interactions and including some things that independent of backend. For example, it provides the zoom_factory discussed in #18159 as well as some more idiosyncratic stuff such as the scalar figsize argument discussed in: #18244

The best single set of examples lives in https://mpl-interactions.readthedocs.io/en/latest/Gallery.html

Also for posterity: mega credit is due to @samanthahamilton who was super helpful in creating and improving the docs!

PR Checklist

• na Has Pytest style unit tests
• na Code is Flake 8 compliant
• na New features are documented, with examples if plot related
• (I hope, I can't even get master to build locally) Documentation is sphinx and numpydoc compliant
• na Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• na Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way

I tried to build the docs to check for sphinx errors but I can't even get master to build :( so here's hoping

For the animation, I followed numpngw and made an apng. For full transparency here's the life course of that animation:

1. Peek to create a gif
2. ezgif.com to convert to apng
3. compress with TinyPng