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