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/platform_introduction.rst b/docs/platform/platform_introduction.rst
new file mode 100644
index 0000000..1e215e9
--- /dev/null
+++ b/docs/platform/platform_introduction.rst
@@ -0,0 +1,70 @@
+###################
+Supported Platforms
+###################
+
+    - Cortex-M85 system:
+
+        - `Corstone-310 Ethos-U55 FVP.
+          <https://arm-software.github.io/AVH/main/simulation/html/Using.html>`_
+
+    - Cortex-M55 system:
+
+        - `FPGA image loaded on MPS3 board (AN547).
+          <https://developer.arm.com/tools-and-software/development-boards/fpga-prototyping-boards/download-fpga-images>`_
+        - `FPGA image loaded on MPS3 board (AN552).
+          <https://developer.arm.com/tools-and-software/development-boards/fpga-prototyping-boards/download-fpga-images>`_
+        - `Corstone-300 Ethos-U55 Ecosystem FVP.
+          <https://developer.arm.com/tools-and-software/open-source-software/arm-platforms-software/arm-ecosystem-fvps>`_
+
+
+    - Cortex-M33 system:
+
+        - `FPGA image loaded on MPS2 board (AN521).
+          <https://developer.arm.com/products/system-design/development-boards/cortex-m-prototyping-systems/mps2>`_
+        - `Fast model FVP_MPS2_AEMv8M (AN521).
+          <https://developer.arm.com/products/system-design/fixed-virtual-platforms>`_
+        - `Musca-B1 test chip board.
+          <https://developer.arm.com/products/system-design/development-boards/iot-test-chips-and-boards/musca-b-test-chip-board>`_
+        - `Musca-S1 test chip board.
+          <https://developer.arm.com/tools-and-software/development-boards/iot-test-chips-and-boards/musca-s1-test-chip-board>`_
+        - `FPGA image loaded on MPS3 board (AN524).
+          <https://developer.arm.com/tools-and-software/development-boards/fpga-prototyping-boards/mps3>`_
+        - `LPC55S69.
+          <https://www.nxp.com/products/processors-and-microcontrollers/arm-microcontrollers/general-purpose-mcus/lpc5500-cortex-m33/lpcxpresso55s69-development-board:LPC55S69-EVK>`_
+        - `B-U585I-IOT02A.
+          <https://www.st.com/en/evaluation-tools/B-U585I-IOT02A.html>`_
+        - `NUCLEO L552ZE Q.
+          <https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-nucleo-boards/nucleo-l552ze-q.html>`_
+        - `DISCO L562QE.
+          <https://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-mpu-eval-tools/stm32-mcu-mpu-eval-tools/stm32-discovery-kits/stm32l562e-dk.html>`_
+        - `nRF9160 DK (nordic_nrf/nrf9160dk_nrf9160).
+          <https://www.nordicsemi.com/Software-and-tools/Development-Kits/nRF9160-DK>`_
+        - `nRF5340 DK (nordic_nrf/nrf5340dk_nrf5340_cpuapp).
+          <https://www.nordicsemi.com/Software-and-tools/Development-Kits/nRF5340-DK>`_
+        - `BL5340 DVK (lairdconnectivity/bl5340_dvk_cpuapp).
+          <https://www.lairdconnect.com/wireless-modules/bluetooth-modules/bluetooth-5-modules/bl5340-series-multi-core-bluetooth-52-802154-nfc-modules>`_
+
+    - Cortex-M23 system:
+
+        - `FPGA image loaded on MPS2 board (AN519).
+          <https://developer.arm.com/products/system-design/development-boards/cortex-m-prototyping-systems/mps2>`_
+        - `M2351.
+          <https://www.nuvoton.com/products/iot-solution/iot-platform/numaker-pfm-m2351/>`_
+        - `M2354.
+          <https://www.nuvoton.com/board/numaker-m2354/>`_
+
+    - Dual Core Cortex-M system:
+
+        - `PSoc64.
+          <https://www.cypress.com/documentation/product-brochures/cypress-psoc-64-secure-microcontrollers>`_
+
+    - Secure Enclave system:
+
+        - :doc:`Musca-B1 Secure Enclave. </platform/arm/musca_b1/secure_enclave/readme>`
+
+    - Runtime Security Subsystem (RSS):
+
+        - :doc:`RSS. </platform/arm/rss/readme>`
+
+The document :doc:`Platform Deprecation and Removal </integration_guide/platform/platform_deprecation>`
+lists the deprecated platforms, removed from the upstream.