Major rewrite of documentation

[tskisner]

• Reorganization of existing content
• Include API docs for nearly all Python interfaces
• Add tutorials based on workshop content
• Other small cleanups such as copyright years

[tskisner]

This PR looks huge because it updates the copyright years to 2020 in all files. It basically just includes work on documentation, and I will self-merge after tests pass before proceeding to tag 2.3.6.