AppNavigator documentation

[RazvanRotaru]

When the final design for the routing is ready, please document it in CONTRIBUTING.md. It's important to have some instructions about how to properly create a new page/route (not in this PR).

(it may also be worth introducing some automated checks, e.g. if someone uses Navigator instead of AppNavigator to refer them to the routing documentation; and on that note -- what happens if e.g. someone does use AppNavigator but didn't implement a route in this file? we should try to make the error message informative so people don't get stuck on this)

Originally posted by @IoanaAlexandru in #265 (comment)