[meta] docs

[sneakers-the-rat]

Standing issue to track and consolidate documentation requests. Add additional requests by commenting below <3

• - @sneakers-the-rat - Make sphinx/readthedocs: there are enough common questions that it seems like it would be helpful to have the existing documentation in the readme/etc. properly split out to a separate docs page along with API docs. As is we only have the top-level DLCLive and Processor usage documented, but have docstrings for most other objects so this seems like an easy win
• - Difference between init_inference and get_pose methods #54 - Difference between init_inference and get_pose
• - Output of dlc.getpose when object is not in view #77 - Structure of numpy array returned from get_pose
• - Installing dlc-live on nvidia jetson #69 - (Deprecated) python 3.6 installation
• - getting labeled frames from dlclive #75 - usage of benchmark function (from a "how-to" index page in addition to summary of what's in the benchmark module?)

(other maintainers feel free to edit this^)