Docs: Structure the Design documents

- gather documents in folders:
   "booting" for secure boot related designs
   "services" for Secure services and SPM
   "software" for general TF-M framework design and rules
- add short document names in indexes

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I4efd309c186e431cf24c3259a35bfef53e2eb24c
diff --git a/docs/design_docs/dual-cpu/index.rst b/docs/design_docs/dual-cpu/index.rst
index f302748..008a244 100644
--- a/docs/design_docs/dual-cpu/index.rst
+++ b/docs/design_docs/dual-cpu/index.rst
@@ -3,9 +3,12 @@
 
 .. toctree::
     :maxdepth: 1
-    :glob:
 
-    *
+    Booting                  <booting_a_dual_core_system.rst>
+    SPE - NSPE communication <communication_prototype_between_nspe_and_spe_in_dual_core_systems.rst>
+    Mailbox                  <mailbox_design_on_dual_core_system.rst>
+    Mailbox update           <mailbox_ns_agent_update.rst>
+    Memory Access Check      <tfm_multi_core_access_check.rst>
 
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