This directory is used to enable website redirects for retired podlings (see also INFRA-21451).
When a podling retires, create a directory with the host name of the podling. e.g. if the podling website is abcd.apache.org, then create directory abcd/
Empty directories are ignored when checking out Git repos, so there needs to be a file in it, so create a bare index.html file. (There's nothing to see, so no point in generating a listing)
Once the Incubator website has been deployed, the website http://abcd.apache.org/ will redirect to https://incubator.apache.org/projects/abcd.html which is the standard status page for podling “abcd” and should mention the “retired” status of the podling.
Note: older podlings with websites of the form http://abcd.incubator.apache.org/ or http://incubator.apache.org/abcd/ are handled by the .htaccess file here: https://github.com/apache/incubator/blob/master/assets/.htaccess