docs: cross-fade between pages with view transitions

[MeAkib]

Navigating between pages in the docs app swapped the content instantly. This enables the router's withViewTransitions(), so browsers that support the View Transitions API cross-fade from the old page to the new one. Other browsers keep the current behavior.

Changes:

• Add withViewTransitions({skipInitialTransition: true}) to the router config. The first page load isn't animated, since there's no previous page to transition from.
• Add a global prefers-reduced-motion: reduce rule that disables the view transition animations, so those users get an instant page swap.

Checked in Chrome against the dev server, comparing against a run with document.startViewTransition removed to get the behavior without transitions:

• Clicking a category card runs a 250ms cross-fade of the root.
• The first page load doesn't start a transition.
• Focus still moves to the new page's focusOnNavigation target.
• Navigating to another page with a #fragment still scrolls the target heading into view, at the same position as without transitions.
• With reduced motion, the transition starts but runs no animations.

[crisbeto]

Looks like there's a lint error.