Create a firmware folder that contains a firmware-specific user guide
extracted from the main user guide including all the information about
firmware workloads like Ubuntu and busybox boot.

Also add a firmware-specific changelog.rst and add a Firmware Boot part
inside the readme.rst#Getting Started section which will be the new
starting point for users.

Change-Id: Ia3c6cff82913c4e1d92d70d8ff25302977bfb319
Signed-off-by: Faiz Abbas <faiz.abbas@arm.com>