doc: De-duplicate readme and license files

The readme.rst and license.rst files in the project root overlap
with the index.rst and license.rst files in the docs/ folder. We
need to use the latter when building the documentation, as Sphinx
requires all included files to be under a common root. However,
the files in the root are currently used by the cgit and Github
viewers.

Using symlinks in Git presents some difficulties so the best
course of action is likely to leave these files but in stub form.

The license.rst file in the root will simply tell the reader to
refer to docs/license.rst.

The readme.rst file will contain a small amount of content that
is derived from the docs/index.rst file, so that the Github main
page will have something valid to show, but it will also contain
a link to the full documentation on ReadTheDocs.

Change-Id: I6dc46f08777e8d7ecb32ca7afc07a28486c9f77a
Signed-off-by: Paul Beesley <paul.beesley@arm.com>