Docs: Added link to security center

This patch adds an external link for the
TF-M Security Center to the `Contributing` and
`Quick Links` sections of the documentation

Change-Id: I5cfed23fcdfaeead35f4050004499e5f6756300c
Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/docs/contributing/index.rst b/docs/contributing/index.rst
index f21a1c7..7b921f2 100644
--- a/docs/contributing/index.rst
+++ b/docs/contributing/index.rst
@@ -5,9 +5,10 @@
     :maxdepth: 1
     :caption: Contents
     :glob:
-    :numbered:
 
     *
+    Security Center <https://developer.trustedfirmware.org/w/collaboration/security_center>
+
 
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