Add handler for `/` to direct to documentation or information page

[jrconlin]

Per recommendation in bug https://bugzilla.mozilla.org/show_bug.cgi?id=1576919, we should add a handler to direct users who visit the server root / to a page that contains documentation or information about this service.

For autopush and autoendpoint, it may be best to direct them to https://mozilla-push-service.readthedocs.io/en/latest/