This change adds the pieces needed to generate docs for the new webhook

resource. Specifically, it adds new doc entries for webhook-list and webhook,

links in the index, and a fixture to auto-generate examples from.
While I was doing this, I encountered lots of trouble because the types for the

apply_to, encoding, and type fields weren't correct (they were lists of

specific strings rather than just being a generic six.text_type). This was

causing the docs to fail to build in a cryptic way as the webapi-resource

sphinx class tried to import "a.ll"

Build docs and looked at the results.