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/ff_isolation.rst b/docs/design_docs/ff_isolation.rst
index e637c65..ad0000e0 100644
--- a/docs/design_docs/ff_isolation.rst
+++ b/docs/design_docs/ff_isolation.rst
@@ -175,7 +175,7 @@
 There is re-usable code like the C-Runtime and RoT Service API which are same
 across different partitions. TF-M creates a Secure Partition Runtime Library
 (SPRTL) as a specific library shared by the Secure Partition. Please refer to
-:doc:`Secure Partition Runtime Library <tfm_secure_partition_runtime_library>`
+:doc:`Secure Partition Runtime Library </design_docs/services/tfm_secure_partition_runtime_library>`
 for more detail.
 
 .. note::
@@ -390,7 +390,7 @@
 
 
 The design document
-:doc:`TF-M Hardware Abstraction Layer <hardware_abstraction_layer>`
+:doc:`TF-M Hardware Abstraction Layer </design_docs/software/hardware_abstraction_layer>`
 gives a detail design, include the platform initialization, isolation
 interfaces. Please refer to it for more detail.