Expand deployments document structure

To cope with the growing number of deployments, the documentation
structure for deployments has been expanded to include sub-pages
for the different types of deployment. This commit includes
fuller descriptions of SP deployments.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Id383cdb67f9e1e3a515e81673cc84077f77b5562
diff --git a/docs/developer/deployments/index.rst b/docs/developer/deployments/index.rst
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+Deployments
+===========
+In the context of the Trusted Services project, a deployment represents a build of an
+assembly of components that is intended to run within a specific environment. Some
+deployments may be built for different platforms using platform specific components
+if needed. The concept of a deployment is general purpose and can be applied to building
+a wide range of targets such as secure partition images, user-space tools, shared libraries
+and test executables.
+
+Supported deployments are described on the following pages:
+
+.. toctree::
+    :maxdepth: 1
+
+    secure-partitions
+    test-executables
+    libraries
+    tools
+    demo-apps
+
+Related deployments:
+
+  - :ref:`Project Structure`
+  - :ref:`Portability Model`
+  - :ref:`Build Instructions`
+  - :ref:`Running Tests`
+
+--------------
+
+*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause