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/index.rst b/docs/design_docs/index.rst
index e091bbb..1c9fcdb 100644
--- a/docs/design_docs/index.rst
+++ b/docs/design_docs/index.rst
@@ -2,11 +2,20 @@
 ================
 
 .. toctree::
-    :maxdepth: 1
-    :glob:
+    :maxdepth: 2
 
-    */index
-    *
+    Secure Boot     <booting/index>
+    Dual CPU        <dual-cpu/index>
+    Secure Services <services/index>
+    Software Design <software/index>
+
+.. toctree::
+    :maxdepth: 1
+
+    Isolation Rules            <ff_isolation.rst>
+    Builtin Keys               <tfm_builtin_keys.rst>
+    Logging system             <tfm_log_system_design_document.rst>
+    Physical Attack Mitigation <tfm_physical_attack_mitigation.rst>
 
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