doc: Move tools info to Getting Started chapter

The Getting Started chapter will hold information on
requirements and on performing initial set-up and build
activities.

Change-Id: Iacf14a049b20f59c84aeaefa1271255ed60a8a5b
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>