Basic NUX states for Phriction
Summary: Adds a basic nux for / and not found documents. Ref T10023
Test Plan: Visit a clean install, see Welcome page. Visit a non built page, see not found UX
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T10023
Differential Revision: https://secure.phabricator.com/D14854