PR Summary

Currently clicking the widgets examples link on the sample plots page puts you at the top of the page rather than the at the widgets examples section. This changes it so you are placed at the correct section.
You can find the offending link here: https://matplotlib.org/3.3.1/tutorials/introductory/sample_plots.html#gui-widgets

which then leads to here:
https://matplotlib.org/3.3.1/gallery/index.html

instead of
https://matplotlib.org/3.3.1/gallery/index.html#widgets

PR Checklist

• na Has pytest style unit tests (and pytest passes).
• na Is Flake 8 compliant (run flake8 on changed files to check).
• na New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• na Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• na New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• na API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).