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/software/index.rst b/docs/design_docs/software/index.rst
new file mode 100644
index 0000000..45240ed
--- /dev/null
+++ b/docs/design_docs/software/index.rst
@@ -0,0 +1,16 @@
+#####################
+Software Design Notes
+#####################
+
+.. toctree::
+    :maxdepth: 1
+
+    Code Sharing               <code_sharing.rst>
+    Hardware Abstraction Layer <hardware_abstraction_layer.rst>
+    Cooperative Scheduling     <tfm_cooperative_scheduling_rules.rst>
+    Code Templates             <tfm_code_generation_with_jinja2.rst>
+    Implicit Typecasintg       <enum_implicit_casting.rst>
+
+--------------
+
+*Copyright (c) 2023, Arm Limited. All rights reserved.*