[docs/code_ex] Updated steps and doc around `routes.json` in exs, removed one ex, updated another.

The steps in the example now use `app.js` for routing and sections regarding routing have been updated to discuss how to define routing paths in `app.js`. In addition, the binding example was updated so that it doesn't use the Url addon, which requires `routes.json`, and the generating_urls example was deleted as was its test.