Add commercial entry to nav bar

Author: FabioLuporini

_config.yml

@@ -64,12 +64,12 @@ navigation:
 - title: FAQ
   url: https://github.com/devitocodes/devito/wiki/FAQ
 
-- title: Twitter
-  url: https://twitter.com/DevitoCodes
-
 - title: Contact us
   url: /contact
 
+- title: Commercial
+  url: https://www.devitocodes.com
+
 include:
     - _static