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>
+
 
 --------------
 
diff --git a/docs/index.rst b/docs/index.rst
index 06bfdcd..59dfb30 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -96,6 +96,7 @@
     :hidden:
     :caption: Quick Links
 
+    Security Center <https://developer.trustedfirmware.org/w/collaboration/security_center>
     docs/reference/changelog
     docs/reference/glossary
     docs/contributing/lic
diff --git a/docs/reference/index.rst b/docs/reference/index.rst
index 02c1077..bb9cf6a 100644
--- a/docs/reference/index.rst
+++ b/docs/reference/index.rst
@@ -4,7 +4,6 @@
 .. toctree::
     :maxdepth: 1
     :glob:
-    :numbered:
 
     changelog
     glossary