[FEATURE] Drop '/index' from generated URLs

The default URLs that are generated for a page include '/index' at the end if the underlying file is called 'index.md'. In my opinion, this is unnecessary and ugly. I'd rather have a link called '/projects' than a link called '/projects/index'. Both would result in the same page anyway, so it's safe to drop them here for cosmetic reasons.
My original thought was that all code of this method could go into `getUrl()` and URL-generation isn't triggered in the constructor. Anyway, at least the method should be protected to allow easy refactoring later, `getUrl()` already offers public access.