List pages without (topic) parent

Another maintenance page :)

* Introduces https://developer-local.allizom.org/fr/docs/without-parent to list where localized documents have no parent set to an English document (translation association lost)
* Introduces https://developer-local.allizom.org/en-US/docs/without-parent to list top-level pages under en-US (those living directly under docs/)
* I sorted routes in urls.py a bit, because I wanted to have a better overview of our special/maintenance pages.
* I commented out the /category/ route there, as I think we do not use this feature (Kitsune inheritance)