doc: certificate format

Change-Id: I9d6427ed5e5fab159b5f96fab8b977b4371f26c3
Refs: #2861
diff --git a/docs/index.rst b/docs/index.rst
index 5543840..eaa0cb7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -33,6 +33,7 @@
    + :doc:`tutorials/utils-ndn-regex`
    + :doc:`tutorials/security-validator-config`
    + :doc:`tutorials/signed-interest`
+   + :doc:`tutorials/certificate-format`
 
 - :doc:`manpages`