Add a Sinatra app

So. Some history:

The service manual app is currently served as HTML compiled in the design principles app. This means that every update to the service manual results in a big diff of HTML peppered with asset cachebusting using URL parameters. This is no fun for anyone.

This pull request:

- Adds a small Sinatra app that serves the service manual, including search
- Adds a Rakefile to register with the router and index content

This branch is currently deployed to preview.