Fix FWU documentation

This patch fixes the followings:
  - the fwu-app deployment has been renamed by an earlier patch and
    this change was not reflected in the documentation
  - components/media/disk/disk_images contains a readme.md file, which
    is converted to reST format and added to the documentation.
  - there are some minor changes in FWU service description related to
    document formatting too

Change-Id: I499959fab1f548bb797974118dda861c3e2f6ea9
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/docs/services/fwu/fwu-gpt-disk-image.rst b/docs/services/fwu/fwu-gpt-disk-image.rst
new file mode 100644
index 0000000..f83c4dd
--- /dev/null
+++ b/docs/services/fwu/fwu-gpt-disk-image.rst
@@ -0,0 +1,6 @@
+.. include:: ../../../components/media/disk/disk_images/readme.rst
+
+..
+    *Copyright (c) 2023, Arm Limited and Contributors. All rights reserved.*
+
+    SPDX-License-Identifier: BSD-3-Clause
\ No newline at end of file