Define docs structure for environments

Adds an extensible structure for documents related to secure processing
environments. The only class of environment currently represented is
Secure Partitions. This change adds a structure to support general
FF-A documents as well as op-tee and hafnium specific documents.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Ic2fdd792580d711091435716e4b9c863df424149
diff --git a/docs/environments/secure-partitions/ff-a/index.rst b/docs/environments/secure-partitions/ff-a/index.rst
new file mode 100644
index 0000000..5c7dc03
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+Firmware Framework for Armv8-A
+==============================
+
+|FF-A| is a standard which *"describes interfaces that standardize communication between the various software images. This
+includes communication between images in the Secure world and Normal world."*
+
+Trusted Services is the home of the FF-A S-EL0 Secure Partitions implementing PSA functionality. The component :ref:`libsp`
+captures helpful abstractions to allow easy FF-A compliant S-EL0 SP development. S-EL0 SPs are SPMC agonistic and can be used
+with an SPMC running in any higher secure exception level (S-EL1 - S-EL3). Currently the solution is tested with an SPMC
+running at S-SEL1 integrated into OP-TEE OS.
+
+--------------
+
+*Copyright (c) 2021-2022, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause