doc: Change structure of the Quickstart FVP_Base section

This patch adds a `Build` and a `Run` section to the quickstart for
FVP-Base. It also changes the instructions to work with the added `Run`
section.

Issue-Id: SCM-3031
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: Iab37bc7d9cec5dd16fce4ef6d3334c0ea1efacba