readme: Move index page content to README

Move the introduction content from the index.rst to the README.rst so
that it can also be found quickly from the top level.

Include the README.rst directly into the index.rst to continue serving
it as the front page material.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/meson.build

@@ -41,6 +41,7 @@ endif
 
 if sphinx.found()
     docs_sources = [
+       '../README.rst',
         'coding-style.rst',
         'conf.py',
         'contributing.rst',