Update specifications section

Update the documentation on supported specifications:
  - clean up PSA related info.
  - add a page on FF-A

Change-Id: I68e8e7762bdce765f38acf503d1d762deb104cc7
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/docs/standards/index.rst b/docs/standards/index.rst
index 486cac0..3ea4651 100644
--- a/docs/standards/index.rst
+++ b/docs/standards/index.rst
@@ -5,7 +5,8 @@
     :maxdepth: 1
     :caption: Contents:
 
-    psa_certification
+    psa
+    ff-a
 
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