doc: Move content to new About chapter

This new chapter is created to hold project-related info
such as methods of contact, supported platforms, and so on.

Change-Id: Ifece3a8d9ba7f7862416a0c81a8707833da4aec7
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>