[streams] Reorganize examples (#255)

The examples are overwhelming and misplaced as-is.

- [ ] Move all examples of creating streams to an appendix
- [ ] Flesh out examples of using streams to make sure we capture the essence
- [ ] Reduce complexity of creating-stream examples. One major potential simplification is to wrap underlying sources/sinks that use promises instead of callbacks.
- [ ] Consider placing examples of method usage inside the section for that method?
- [ ] Add examples of creating and using strategies to each strategy's section.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/255