Create a common folder consisting of prerequisites.rst,
board-setup.rst, troubleshooting-guide.rst, fvp-features.rst,
standalone-baremetal-readme.rst, toolchain-readme.rst and
vulnerability-reporting.rst which are meant to be shared among/referred
to by all the project components

Change-Id: I06bbd2e9efb963ca49209535840c63017239318e
Signed-off-by: Faiz Abbas <faiz.abbas@arm.com>