add host-based routing docs

[WhiteApfel]

Yesterday I wanted to use host-based routing, saw the necessary methods in the Starlette class, but did not find any info in docs. After searching, I found #363 and decided to add at least a minimal mention of host-based routing to the documentation.

#369 opened almost three years ago and is still open 🤡

[aminalaee]

...
routes = [
     Host('api.example.org', api)
 ]
 app = Starlette(routes=routes)
 app.host('www.example.org', site)
 news_host = Host('news.example.org', news)
 app.router.routes.append(news_host)

I think this is much better. Do you think we may need a little explanation here?

[WhiteApfel]

There are several ways to connect host-based routes to your application

It'll do? I think that this does not make much sense, because the meaning from the upper sections is duplicated

[aminalaee]

That makes sense.

[aminalaee]

Thank you for this.