docs: clarify and cleanup the certificate format spec

Also, cleanup references to the legacy signed interest spec

Change-Id: Ib761399b6e44cefea6331e4e128bb2976c2fc11d
diff --git a/docs/conf.py b/docs/conf.py
index a2df0fc..22f4bc1 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -18,7 +18,7 @@
 # -- Project information -----------------------------------------------------
 
 project = u'ndn-cxx: NDN C++ library with eXperimental eXtensions'
-copyright = u'Copyright © 2013-2020 Regents of the University of California.'
+copyright = u'Copyright © 2013-2021 Regents of the University of California.'
 author = u'Named Data Networking Project'
 
 # The short X.Y version.