Create an explainer

[anssiko]

The Vibration API predates the explainer-era, regardless we should consider retroactively creating an explainer document to help the community better understand the API, its use cases and considered alternatives.

Some use cases are discussed in w3ctag/design-reviews#971 and real-world examples include e.g. Jackbox Games and Construct via https://github.com/web-platform-tests/interop/issues?q=Vibration