Developer doc index page points to correct API docs

The index.html page of our developer web docs should now
reference the auto-generated API documentation, and we can
remove the out-of-date hand-written api-spec-v1.rst.
It served us well early on, but is impractical to keep up
to date with a fast-moving project.

Change-Id: Ie71f43ef363501d4c470485a011267755f41c5b3
Merge "Developer doc index page points to correct API docs"