PR Summary

Based on https://matplotlib.org/stable/devel/documenting_mpl.html#section-formatting section titles should be Upper lower. As I've been confused about what the standard is (not reading the above link and rather relying on the templates), I thought it made sense to change the templates.

I also changed the few next User What's New (skipped the API ones).

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).