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/platform/index.rst b/docs/platform/index.rst
new file mode 100644
index 0000000..4c53ebb
--- /dev/null
+++ b/docs/platform/index.rst
@@ -0,0 +1,55 @@
+##############
+TF-M Platforms
+##############
+
+.. toctree::
+    :maxdepth: 2
+    :caption: arm
+
+    arm/index
+
+.. toctree::
+    :maxdepth: 1
+    :caption: NXP
+    :glob:
+
+    nxp/**
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Cypress
+    :glob:
+
+    cypress/psoc64/index
+
+.. toctree::
+    :maxdepth: 2
+    :caption: STM
+    :glob:
+
+    stm/index
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Nordic
+    :glob:
+
+    nordic_nrf/**
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Laird Connectivity
+    :glob:
+
+    lairdconnectivity/**
+
+.. toctree::
+    :maxdepth: 1
+    :caption: Nuvoton
+    :glob:
+
+    nuvoton/**
+
+--------------
+
+*Copyright (c) 2020-2021, Arm Limited. All rights reserved.*