subtitle kwarg to set_titles_and_labels

[jukent]

PR Summary

Adds a subtitle keyword argument to set_titles_and_labels by leveraging the fig.suptitle() function
Closes #52

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst in a relevant section for the next unreleased release. Possible sections include: Documentation, New Features, Bug Fixes, Internal Changes, Breaking Changes/Deprecated
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
• Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.

Documentation

• Docstrings have been added to all new functions
• Docstrings have been updated with any function changes
• Internal functions have a preceding underscore (_) and have been added to docs/internal_api/index.rst
• User facing functions have been added to docs/user_api/index.rst under their module

Examples

• Any new notebook examples added to docs/examples/ folder
• Clear all notebook cells
• New notebook files added to docs/examples.rst toctree
• New notebook files added to new entry in docs/gallery.yml with appropriate thumbnail photo in docs/_static/thumbnails/

[kafitzgerald]

Looks good!

Should we add this to the usage example as well?

[jukent]

Sure thing

[kafitzgerald]

There's something funky going on still with the example:
https://geocat-viz--185.org.readthedocs.build/en/185/examples/set_titles_and_labels.html

Looks like a minor typo