Adding "Getting Started" documentation section.

Adds a new high-level introduction section to docs with several diagrams that help explain the intent of the library.
I'm not a fan of the "deep link" terminology or the discussion about the document fragment. The document fragment isn't exactly right and reminds me of a [DocumentFragment](https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment) and the bit about "when the deep link changes" is confusing... changes the resource location?

The only thing the router does in response to a changed URL is trigger a route. What's done in that route is incidental to updating the model.