Document hints and add the changelog entry

A Hints guide in the docs Introduction section with a live sample
component (self-contained mock UI the beacons attach to), covering the
close-vs-dismiss semantics, the localStorage persistence recipe, beacon
placement/styling and the full options and methods reference. The docs
app gains driver.js/hints and hints.css source aliases for HMR, same as
the playground.

The Unreleased changelog section lists the hints entry, the new CSS
variables, the sideEffects field, the shared popover layer and the
tsdown build migration — which also makes the release flow green: the
release aborts on an empty Unreleased section.