Update Contributing docs page

[rclune]

• Add information about docstring formatting (which style is needed for the API docs to work?)
• Link to external resources about writing tests
• Provide information about where tests should go in the repo and how/when they should get automatically run with the GitHub runners
• "Keep commits as 'one logical unit.' (Easy to do in VSCode)" - this comment would be more useful with an example or a link to something that expands on this idea
• Link to the PR checklist
• Add a documentation section that tells contributors how to build the documentation locally, links out to Sphinx tutorials, etc.
• Unclear what the 'Visual Aids' comic is trying to get across IMO
• Update PR Hygiene section - folks should be creating PRs to merge with the staging branch, right?
• How does one contact the maintainers? Who should be contacted?
• Link to the README (bottom of this page)