doc: Convert internal links to RST format

Currently links between documents are using the format:

<path/to/><filename>.rst

This was required for services like GitHub because they render each
document in isolation - linking to another document is like linking
to any other file, just provide the full path.

However, with the new approach, the .rst files are only the raw
source for the documents. Once the documents have been rendered
the output is now in another format (HTML in our case) and so,
when linking to another document, the link must point to the
rendered version and not the .rst file.

The RST spec provides a few methods for linking between content.
The parent of this patch enabled the automatic creation of anchors
for document titles - we will use these anchors as the targets for
our links. Additional anchors can be added by hand if needed, on
section and sub-section titles, for example.

An example of this new format, for a document with the title
"Firmware Design" is :ref:`Firmware Design`.

One big advantage of this is that anchors are not dependent on
paths. We can then move documents around, even between directories,
without breaking any links between documents. Links will need to be
updated only if the title of a document changes.

Change-Id: I9e2340a61dd424cbd8fd1ecc2dc166f460d81703
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/components/firmware-update.rst b/docs/components/firmware-update.rst
index 30bdc24..2bff00f 100644
--- a/docs/components/firmware-update.rst
+++ b/docs/components/firmware-update.rst
@@ -14,8 +14,8 @@
 
 FWU implements a specific part of the Trusted Board Boot Requirements (TBBR)
 specification, Arm DEN0006C-1. It should be used in conjunction with the
-`Trusted Board Boot`_ design document, which describes the image authentication
-parts of the Trusted Firmware-A (TF-A) TBBR implementation.
+:ref:`Trusted Board Boot` design document, which describes the image
+authentication parts of the Trusted Firmware-A (TF-A) TBBR implementation.
 
 Scope
 ~~~~~
@@ -53,10 +53,11 @@
    at other Exception Levels.
 #. Export a platform interface to provide FWU common code with the information
    it needs, and to enable platform specific FWU functionality. See the
-   `Porting Guide`_ for details of this interface.
+   :ref:`Porting Guide` for details of this interface.
 
 TF-A uses abbreviated image terminology for FWU images like for other TF-A
-images. An overview of this terminology can be found `here`_.
+images. See the :ref:`Image Terminology` document for an explanation of these
+terms.
 
 The following diagram shows the FWU boot flow for Arm development platforms.
 Arm CSS platforms like Juno have a System Control Processor (SCP), and these
@@ -70,8 +71,8 @@
 Each FWU image and certificate is identified by a unique ID, defined by the
 platform, which BL1 uses to fetch an image descriptor (``image_desc_t``) via a
 call to ``bl1_plat_get_image_desc()``. The same ID is also used to prepare the
-Chain of Trust (Refer to the `Authentication Framework Design`_
-for more information).
+Chain of Trust (Refer to the :ref:`Authentication Framework & Chain of Trust`
+document for more information).
 
 The image descriptor includes the following information:
 
@@ -394,11 +395,6 @@
 
 *Copyright (c) 2015-2019, Arm Limited and Contributors. All rights reserved.*
 
-.. _Trusted Board Boot: ../design/trusted-board-boot.rst
-.. _Porting Guide: ../getting_started/porting-guide.rst
-.. _here: ../getting_started/image-terminology.rst
-.. _Authentication Framework Design: ../design/auth-framework.rst
 .. _Universally Unique Identifier: https://tools.ietf.org/rfc/rfc4122.txt
-
 .. |Flow Diagram| image:: ../resources/diagrams/fwu_flow.png
 .. |FWU state machine| image:: ../resources/diagrams/fwu_states.png