Doc: add a section describing target platforms

Extend the documentation with pages describing what is a target
platform, supported categories, deprecation process, etc...

Change-Id: I92c2ddd84a0387474d09ca0843d2c92c01106884
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index c6cd80a..1135ba7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -80,6 +80,7 @@
    security/index
    deployments/index
    certification/index
+   target-platforms/index
 
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