Update docs with Starlette 0.13+ usage

[florimondmanca]

Starlette 0.13 introduced a new declarative style for defining apps, routes, etc. (see

routes = [
    Route('/', homepage),
    Mount('/users', routes=[
        Route('/', users, methods=['GET', 'POST']),
        Route('/{username}', user),
    ])
]

app = Starlette(routes=routes)

Our documentation is now out of date w.r.t. https://www.starlette.io. So although things aren't broken, we need to update the code snippets to use the declarative style for consistency with the newest Starlette release.

A note on per-endpoint caching docs: we should keep the @cached() decorator on top of HTTPEndpoint instances, even though #23 makes me think we should probably rejig the @cached() decorator into something that can perform both as a decorator and a standard function (e.g. cache_endpoint(home, cache=cache)).