fix difference between packaged version and master

@DeepDiver1975 described the problem for packaging ownCLoud to me:

On master there is the index.php in the doc subfolder but on a packaged ownCloud (read releases) the documentation is placed in there and the index file is a index.html. This PR fixes the difference between packaged version and git master.

Supersedes #4470 

cc @jancborchardt @Raydiation @zeppel