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are no longer valid but can be restored after the TF-A User Guide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/porting/mandatory-mods.rst b/docs/porting/mandatory-mods.rst
index dd4242f..7b94568 100644
--- a/docs/porting/mandatory-mods.rst
+++ b/docs/porting/mandatory-mods.rst
@@ -201,8 +201,8 @@
     of power domains that are its direct children.
 
 The array format is the same as the one used by Trusted Firmware-A and more
-details of its description can be found in the Trusted Firmware-A documentation:
-`docs/psci-pd-tree.rst`_.
+details of its description can be found in the
+`Trusted Firmware-A documentation`_.
 
 Function : tftf_plat_get_mpidr()
 --------------------------------
@@ -301,4 +301,4 @@
 *Copyright (c) 2019, Arm Limited. All rights reserved.*
 
 .. _SP805: https://static.docs.arm.com/ddi0270/b/DDI0270.pdf
-.. _docs/psci-pd-tree.rst: https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/about/docs/psci-pd-tree.rst
+.. _Trusted Firmware-A documentation: https://trustedfirmware-a.readthedocs.io/en/latest/design/psci-pd-tree.html#psci-power-domain-tree-structure