Adds missing content tag

git-svn-id: https://svn.apache.org/repos/asf/struts/site@1565952 13f79535-47bb-0310-9956-ffa450edef68
Reformats configuration and reduces allowed extensions to .md only

git-svn-id: https://svn.apache.org/repos/asf/struts/site@1565953 13f79535-47bb-0310-9956-ffa450edef68