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are no longer valid but can be restored after the TF-A User Guide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/license.rst b/docs/license.rst
index 46124fc..3e940c8 100644
--- a/docs/license.rst
+++ b/docs/license.rst
@@ -1,9 +1,7 @@
 License
 =======
 
-The software is provided under a BSD-3-Clause `license`_. Contributions to this
-project are accepted under the same license with developer sign-off as
-described in the `Contributing Guidelines`_.
+The software is provided under a BSD-3-Clause license.
 
 ::