Reformat README as ReST and include in built docs

I've renamed the README and tweaked its formatting to be valid
ReStructured Text rather than markdown.

This allows it to be included in the built docs; which means our other
docs can link directly to a section of the README (rather than just
telling users to "go find the README somewhere and look for a
heading").

The major changes are slightly different formatting for the headings
and subheadings and sub-sub-headings, and using explicit enumeration
for the lists.

ReST can handle auto-enumeration, or monotonically increasing explicit
enumeration, but when it sees the "1." marker again it assumes a new
list is being started, which gets it confused. I've opted for explicit
enumeration as it ensures the reader gets the same understanding
whether looking at the source or the generated document.

I've tweaked a few of the links to point at the local doc rather than
the docs.o.o version, to improve consistency for someone reading docs
from a particular checkout. I have assumed that anyone who needs to
read the d.o.o versions knows how to find them.

Change-Id: I964fd5228b4f1fb863fb3fd138ba862cad9303e6
Merge "Reformat README as ReST and include in built docs"