Explain the “ScrollSpy” use case on the visibility component’s page

[awgv]

The documentation uses the visibility component to activate sections in the right sidebar when a page is scrolled. The lack of straightforward equivalent to Bootstrap’s “ScrollSpy” confuses some people, and maybe it’s a good idea to add a section to visibility’s examples tab and explain how the component can be used to create a “ScrollSpy”.

For reference: Semantic-Org/Semantic-UI#400.