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/services/index.rst b/docs/design_docs/services/index.rst
new file mode 100644
index 0000000..5024f88
--- /dev/null
+++ b/docs/design_docs/services/index.rst
@@ -0,0 +1,21 @@
+###############
+Secure Services
+###############
+
+.. toctree::
+    :maxdepth: 1
+
+    Secure Partition Manager    <secure_partition_manager.rst>
+    Secure RTL                  <tfm_secure_partition_runtime_library.rst>
+    Inter-Process Communication <tfm_psa_inter_process_communication.rst>
+    Stateless Services          <stateless_rot_service.rst>
+    Service Signing             <tfm_uniform_secure_service_signature.rst>
+    Crypto                      <tfm_crypto_design.rst>
+    Initial Attestation         <symmetric_initial_attest.rst>
+    Internal Storage            <tfm_its_service.rst>
+    Firmware Update             <tfm_fwu_service.rst>
+    PS Key Management           <ps_key_management.rst>
+
+--------------
+
+*Copyright (c) 2023, Arm Limited. All rights reserved.*