Mark `react-page` as unmaintained

We haven't done a great job of staying on top of issues for this project. I want to eventually revisit `react-page` as the *premier* way to develop React components, but right now it's marketed as an easy way to create real-world apps or as an example of server rendering. There are easier, better maintained ways to create real-world apps and simpler examples of server rendering, so I think we should refocus this project as a development environment before encouraging people to use it again.