Refactor top-level document structure

In preparation for extending documentation on PSA certified and
secure processing environments, the top-level structure has been
cleaned-up and simplified. PNGs from tf-m for docs home have
been reused for consistency.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Id2b7d9b46dd90f419d00c417a06003e32862ead0
diff --git a/docs/environments/index.rst b/docs/environments/index.rst
index a244827..18948ea 100644
--- a/docs/environments/index.rst
+++ b/docs/environments/index.rst
@@ -1,16 +1,14 @@
-Processing Environments
-=======================
+Environments
+============
 
 .. toctree::
     :maxdepth: 1
     :caption: Contents:
 
-    ffa/libsp
-    deployment-guides/fvp-deployment-guide.rst
-    deployment-guides/opteesp-deployment-guide.rst
+    secure-partitions/index
 
 --------------
 
-*Copyright (c) 2020-2021, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause