List top level docs and docs without parent

New maintenance pages :)

* 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/top-level to list top-level pages (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 fixed some indentation issues Jannis mentioned in the previous diff. 

Things to consider (as follow-ups?):
0) https://developer-local.allizom.org/en-US/docs/without-parent does not make sense now. As there is no translation source to set for English.
1) There is a "category" route, which I think we are not using at all. We do have categories for docs? I think this might be an inheritance from Kitsune which we could get rid of.
2) "The filter_for_list method "is slowly becoming a kitchen sink and harder to test". Jannis proposes  "multiple model manager methods". I looked at the other managers in that file, but I had no clue how to refactor it that way. Would love to help with that and learn about it in a follow-up.