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/arm/index.rst b/docs/platform/arm/index.rst
new file mode 100755
index 0000000..48a8223
--- /dev/null
+++ b/docs/platform/arm/index.rst
@@ -0,0 +1,18 @@
+#############
+Arm platforms
+#############
+
+.. toctree::
+    :maxdepth: 1
+
+    Corstone-1000 <corstone1000/readme.rst>
+    Corstone-300 FPGA (AN547) <mps3/an547/README.rst>
+    Corstone-300 FPGA and FVP (AN552) <mps3/an552/README.rst>
+    Corstone-310 FVP <mps3/corstone310_fvp/README.rst>
+    Musca-B1 <musca_b1/sse_200/readme.rst>
+    Musca-B1 Secure Enclave <musca_b1/secure_enclave/readme.rst>
+    Runtime Security Subsystem <rss/readme.rst>
+
+--------------
+
+*Copyright (c) 2022, Arm Limited. All rights reserved.*