Docs: Restructure the documents

Restructure the file category to let it more friendly to users.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I7ced0e2d700ce03423e472e0098608f3445ba169
diff --git a/docs/contributing/index.rst b/docs/contributing/index.rst
index 7b921f2..e5f9c66 100644
--- a/docs/contributing/index.rst
+++ b/docs/contributing/index.rst
@@ -1,15 +1,13 @@
-Contributing
-============
+Contribution Guidelines
+=======================
 
 .. toctree::
     :maxdepth: 1
-    :caption: Contents
     :glob:
 
     *
-    Security Center <https://developer.trustedfirmware.org/w/collaboration/security_center>
 
 
 --------------
 
-*Copyright (c) 2020, Arm Limited. All rights reserved.*
+*Copyright (c) 2020-2021, Arm Limited. All rights reserved.*