doc: Update link formats

Use docutils/Sphinx-type linking with the :ref: keyword where
possible, instead of direct links to files.

As part of this patch some external links have been updated too,
especially those that point to the TF-A docs. Some link targets
are no longer valid but can be restored after the TF-A User Guide
is split up.

Change-Id: I328d5141cf9b32471bf669bfc0191e8f6f883a48
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index 25881dd..a956efb 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -12,8 +12,10 @@
 
 This document describes how to build the Trusted Firmware-A Tests (TF-A Tests)
 and run them on a set of platforms. It assumes that the reader has previous
-experience building and running the `Trusted Firmware-A (TF-A)`_.
+experience building and running `Trusted Firmware-A (TF-A)`_.
 
 --------------
 
 *Copyright (c) 2019, Arm Limited. All rights reserved.*
+
+.. _Trusted Firmware-A (TF-A): https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git