Fix Documentation Index

[jamesfredley]

https://apache.github.io/grails-spring-security/snapshot/ghpages.html does not link to the combined documentation or the individual index pages. such as

https://apache.github.io/grails-spring-security/snapshot/acl-plugin/guide/index.html

https://github.com/apache/grails-spring-security/tree/gh-pages/snapshot