[docs] Replace playground Loading text with shimmer skeleton

[ahmedsadid]

What changed / motivation ?

The playground showed a plain "Loading…" text while the editor booted up. This replaces it with a shimmer skeleton that mirrors the playgrounds editor/preview layout, so the loading state previews the eventual UI instead of a bare text label.

• Reworks PlaygroundPlaceholder in DynamicPlayground.tsx into skeleton editor panels (tab pills + code-line placeholders) and a preview panel, with an animated shimmer sweep (stylex.keyframes).
• Uses the existing playground theme variables, so it respects light/dark.
• Adds role="status" / aria-label="Loading playground" for accessibility.

Linked PR/Issues

Part of #1550

Additional Context

tsc, Prettier, ESLint, and waku build all pass. The placeholder is rendered server-side via ClientOnlys fallback, so it appears immediately on load.

	Before	After
Dark
Light

Pre-flight checklist

• I have read the contributing guidelines Contribution Guidelines
• Performed a self-review of my code

[vercel]

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