Show hints with default styling in the docs and add a customization page

The guide's live sample dropped the docs theme so readers see the
default look first. New Styling Hints page under Examples: beacon color
and size through CSS variables and per-beacon classes, static beacons,
custom and hidden dismiss buttons, the overlay focus mode, and popover
theming. HintsSample takes an id prefix and instance config so several
samples share a page.