Docs: Restructure in the Platfoms section

- Move platforms' documentaions from /ext/target/ to the /platform folder
- Name arm platforms in TOC explicitly.
- Merge CS-1000 platform README with Openamp doc
- Rebase and include RSS platform

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: If8bc343e92910e2ce94f1fe1d6c8c81802d63165
diff --git a/docs/introduction/readme.rst b/docs/introduction/readme.rst
index a822508..efffcf9 100644
--- a/docs/introduction/readme.rst
+++ b/docs/introduction/readme.rst
@@ -34,7 +34,7 @@
 secure connection with edge gateways and IoT cloud services. It also protects
 the critical security assets such as sensitive data, keys and certificates on
 the platform. TF-M is supported on several Cortex-M based
-:doc:`Microcontrollers </platform/ext/index>` and Real Time Operating
+:doc:`Microcontrollers </platform/index>` and Real Time Operating
 Systems (RTOS).
 
 Terms ``TFM`` and ``TF-M`` are commonly used in documents and code and both
@@ -60,9 +60,9 @@
 - The ``platform`` folder currently contains platforms support imported from
   the external project and the files may have different licenses.
 
-.. include:: /platform/ext/platform_introduction.rst
+.. include:: /platform/platform_introduction.rst
 
-The document :doc:`Supported Platforms </platform/ext/index>` lists the details.
+The document :doc:`Supported Platforms </platform/index>` lists the details.
 
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 Release Notes and Process