Add @app to the Page Guide

#### Is your feature request related to a problem? Please describe
The `@app('/')` feature is explained in the API section of the docs (https://wave.h2o.ai/docs/api/server#app), but is only really find-able if someone is looking for it. When users are on the Pages section of the Guide (https://wave.h2o.ai/docs/pages/) it might be nice for them to explicitly learn about `@app()` as this is a shortcut for sending apps to a specific page.

#### Describe the solution you'd like
Add a brief sentence and code snippet to the Pages Guide: https://wave.h2o.ai/docs/pages/

Right after the code for `To reference the current page in a Wave app, use q.page.`. Here is a first-pass suggestion


```
To reference a specific page in a Wave app, use the `@app(/foo)` decorator

~
@app('/foo')
async def serve(q: Q):
page = q.page
~

```