docs: add a manpage for ndn-log

This commit also removes reference to non-existing ndnsec-validator.conf

refs: #3918, #4184

Change-Id: I0413a2418db18db07ed9803ed6c9789374a56ded
diff --git a/docs/manpages.rst b/docs/manpages.rst
index 67f69a3..c88228e 100644
--- a/docs/manpages.rst
+++ b/docs/manpages.rst
@@ -3,6 +3,7 @@
 
 .. toctree::
     ndn-client.conf     <manpages/ndn-client.conf>
+    ndn-cxx logging     <manpages/ndn-log>
     manpages/ndnsec
     ndnsec-list         <manpages/ndnsec-list>
     ndnsec-get-default  <manpages/ndnsec-get-default>