Docs: Refine introduction

Refine the content of introduction.
Move 'releases' and 'platform supported' into their separate directory.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: Ia791ae8114401966a9512fb0d12f98068b1b775e
diff --git a/platform/ext/index.rst b/platform/ext/index.rst
index 7fa6b74..3c3e8d1 100644
--- a/platform/ext/index.rst
+++ b/platform/ext/index.rst
@@ -7,10 +7,11 @@
 
     /platform/readme
     readme
+    /platform/ext/platform_introduction.rst
     /platform/ext/platform_deprecation.rst
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: arm
     :glob:
 
@@ -19,42 +20,42 @@
     otp_provisioning.rst
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: NXP
     :glob:
 
     target/nxp/**
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: Cypress
     :glob:
 
     target/cypress/**
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: STM
     :glob:
 
     target/stm/index
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: Nordic
     :glob:
 
     target/nordic_nrf/**
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: Laird Connectivity
     :glob:
 
     target/lairdconnectivity/**
 
 .. toctree::
-    :maxdepth: 4
+    :maxdepth: 1
     :caption: Nuvoton
     :glob: