Move favicon.ico to website root

In order to use Sling's favicon for HTML pages which cannot set a link tag in the head section like javadoc (https://sling.apache.org/apidocs/sling11/index.html) or Maven generated sites (https://sling.apache.org/components/htl-maven-plugin/) the favicon.ico should be moved to the root. Currently https://sling.apache.org/favicon.ico contains the icon of Apache.