Tidy/prettify/segregate the master toctree

At the moment our index.rst is just a list of context-free links.

In order to give people more hints about what each doc is for, I've
split the toctree into three and added some section headings and
notes.

Change-Id: Ie45d5ab6324d4a33b08672bfec00e20a2bac5d34
Merge "Tidy/prettify/segregate the master toctree"