The index page for the API documentation was nothing but a giant

toctree, which didn't really provide any guidance at all. This has been

replaced with a new index page that gives a brief overview of the API,

followed by sections on:

Getting started with the API (containing the overview, authentication,

  and glossary)
API bindings available (linking to the RBTools Python API docs)
Working with special data (currently consisting of rich text fields

  and, later, extra_data handling)
Available resources (linking to the resource tree and categories)
Errors (linking to the full list of errors)

It also gets rid of any mention of the old API 1.0 docs, which isn't

worth including anymore.

Checked for build and spelling errors.
Tested that the links pointed to the right places.