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/index.rst b/docs/index.rst
index add4ec2..17eebd64 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -85,7 +85,7 @@
     :maxdepth: 1
     :hidden:
 
-    Platform Selection<platform/ext/index>
+    Platform Selection<platform/index>
     building/tfm_build_instruction
     integration_guide/index
     technical_references/design_docs/index