Improve documentation

[kallewesterling]

• Set up industry standard package for documentation, e.g. Sphinx
• Set up appropriate file structure for Sphinx, and move notebooks into the documentation (as worked examples)
• Add instructions on how to contribute to the codebase
• Add instructions to the README on how to build the documentation for the codebase (for folks who want to contribute to the documentation, e.g. "How do I add another worked example to the repository?")