doc: Move content out of readme and create new index page

doc: Move content out of readme and create new index page

Previously the readme.rst file served as the entrypoint for the
documentation. With a Sphinx build the top-level document is set
to be index.rst as it contains the primary document index.

This patch moves some content from readme.rst into index.rst and
splits the license information out into license.rst.

Change-Id: I5c50250b81136fe36aa9ceedaae302b44ec11e47
Signed-off-by: Paul Beesley <paul.beesley@arm.com>