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/spm/optee/index.rst b/docs/environments/secure-partitions/spm/optee/index.rst
new file mode 100644
index 0000000..2149880
--- /dev/null
+++ b/docs/environments/secure-partitions/spm/optee/index.rst
@@ -0,0 +1,15 @@
+S-EL0 Secure Partitions under OP-TEE
+====================================
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Contents:
+
+    userspace-programs-on-fvp
+    opteesp-deployment-guide
+
+--------------
+
+*Copyright (c) 2022, Arm Limited and Contributors. All rights reserved.*
+
+SPDX-License-Identifier: BSD-3-Clause