Rewrite documentation in mkdocs

[bdpedigo]

• removes sphinx, adds mkdocs as documentation generator
• adds a new "client" api to the docs to more closely mirror how people interface with the package through a client object and its properties (the sub-clients)
• adds and corrects some type hints (but didn't attempt to catch everything)

[bdpedigo]

preview of what this would look like: https://bdpedigo.github.io/CAVEclient/

[bdpedigo]

I think this is good to go except for changing github paths etc. in mkdocs.yml. will wait to do that until any other feedback is done