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
diff --git a/docs/index.rst b/docs/index.rst
index 3860199..d5ab8fc 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -11,6 +11,7 @@
    process/index
    components/index
    design/index
+   porting-guide
    plat/index
    perf/index
    security_advisories/index
@@ -84,7 +85,7 @@
 
 --------------
 
-*Copyright (c) 2013-2021, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2023, Arm Limited and Contributors. All rights reserved.*
 
 .. _Armv7-A and Armv8-A: https://developer.arm.com/products/architecture/a-profile
 .. _Secure Monitor: http://www.arm.com/products/processors/technologies/trustzone/tee-smc.php