docs(porting): move porting guide upper in table of contents

docs(porting): move porting guide upper in table of contents

The porting guide is currently hosted under the 'Getting started'
section. Yet, porting the full firmware to a new platform is probably
not the first thing that one would do. Before delving into the
details, one would probably start by building the code for an emulated
platform, such as Arm FVP.

Furthermore, the porting guide is such a big and important document
that it probably deserves being visible in the main table of contents.
Thus, move it just above the list of supported platforms.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I51b3d2a93832505ab90d73c823f06f9540e84c77