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/developer/index.rst b/docs/developer/index.rst
index 5fe886f..1fdccae 100644
--- a/docs/developer/index.rst
+++ b/docs/developer/index.rst
@@ -1,5 +1,5 @@
-Developer Docs
-==============
+Developer Documents
+===================
 
 .. toctree::
     :maxdepth: 1
@@ -10,8 +10,6 @@
     portability-model
     service-access-protocols
     service-locator
-    deployments/index
-    service-descriptions/index
     software-requirements
     build-instructions
     running-tests