[CdkScrollable] Add more documentation and demo examples

i'm trying to put together the [test](https://github.com/angular/material2/blob/master/src/cdk/scrolling/scroll-dispatcher.spec.ts) of how to put a "scrollspy" together.
but unfortunately i did not find any help in the [documentation](https://material.angular.io/cdk/scrolling/overview).

can you expect a user manual?
thanks!

Start with the linked scroll-dispatcher.spec.ts test and the CDK scrolling overview to identify the documented CdkScrollable and scrollspy use cases. Done means adding the requested user guidance and demo examples for assembling a scrollspy, with the documentation matching the demonstrated behavior.

索引モデルが issue の本文から書いたものです。