[Frontends] Docs landing page for "Try It Out" with environment and template picker

[mkucmus]

Part of epic #2360.

Introduce a dedicated docs landing page that "Try It Out" routes to, so users pick an environment and template before the sandbox boot begins. This absorbs several first-run UX stories from the epic by setting expectations up front.

Scope

• Location: apps/docs (publishes to frontends.shopware.com)
• Page sections: environment picker, template picker, expected duration, "what to watch during boot", known-warning FAQ
• Environment cards:
  • StackBlitz — fastest to open, noisy boot
  • GitHub Codespaces — clean, slower cold start
  • Local clone — fastest after first setup
• Template cards:
  • vue-starter-template — blank starter
  • vue-starter-template-extended — Lumora demo store
• Each card includes a 1-line pitch and an "Open" button with a deep link (StackBlitz URL, Codespaces URL, git clone command)
• Repoint existing "Try It Out" entry points (root README.md, docs site CTA, any in-repo links) to this new page

Acceptance criteria

• Landing page reachable at a stable path under frontends.shopware.com
• All three environments launchable from the page via deep links
• Both templates selectable
• Expected duration and known-warning FAQ visible without scrolling past the fold on desktop
• All previous "Try It Out" entry points updated to route here