Add guide and samples for getting started

[karashiiro]

Currently, guide-level documentation only exists for specialized features like request/response transformers. For basic usage, the README suggests looking at n0madic's repo, since the Scraper interface is nearly the same.

That decision was mostly made out of laziness — real guide-level documentation should be written, and there should be working examples beyond unit tests, especially for common runtime environments where developers are expected to want to use this library (Node/browser/edge runtimes).