Explainer structure needs to be more compelling

[domenic]

Suggested ordering:

• Introduction that goes straight to the code, i.e. what is currently the "EventTarget integration" section.

• The observable API

  • EventTarget integration: this section can be short because most of it's covered in the intro

  • The Observable object: explain in more detail observable as a primitive, how you create your own

  • Operators and combinators

• Background

  • Where do observables fit in the landscape of async/etc. primitives? (What is currently "Introduction"

  • History

  • Userland libraries

• Concerns

• Standards venue

• Authors