feat: publish the documentation and the card wizard from one Astro site

[marcalexiei]

• Closes Publish documentation from release instead of master #461

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• Turns apps/frontend into an Astro + Starlight site.
  Serves the docs at /frontend/docs and the card wizard at /frontend:
  one dev server, one build, one deploy, and no Python in vercel-preparation.sh or in the local setup.
• The wizard becomes a page of that site: a React island on Starlight's splash layout,
  so it inherits the header, footer and search.
  Its own app bar, theme picker and Redux theme slice go away.
  Starlight writes the data-theme daisyUI already reads, so one control themes both halves.
• The *.md sources move to apps/frontend/src/content/docs/docs/ with a title in
  frontmatter and Starlight's :::note asides, which lets Astro resolve their links and
  images natively (no markdown-conversion code of our own).
  starlight-links-validator then fails the build on a dead internal link.
• packages/core/src/themes/README.md is generated into the site instead; README and CONTRIBUTING links follow the move.
• Tailwind now shares a page with Starlight, so index.css declares the cascade-layer order
  (its utilities must outrank Starlight's reset) and scopes the app's element rules under
  .wizard, leaving the site identical on both halves.

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To try it out just run

pnpm --filter ./apps/frontend run dev

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Wizard	Documentation

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[marcalexiei]

I made few changes with the last commit (totally fine if you want to go ahead without them):

• Splits the "Advanced Customization" page into smaller pages, grouped in the sidebar under two categories:
  • Cards: one page per card
  • Customization: Common Options, Theming, Available Themes, Locales and Aligning Cards
  
• Moved the options shared by every card out from under "GitHub Stats Card", where they were nested despite applying to all of them.
• Redirects /frontend/docs/advanced_documentation/ to "Common Options" and both group paths to their first page, so published URLs do not hit 404.
• Links each card title in the Overview page's "Card Types" section to that card's page.

[martin-mfg]

I haven't completely reviewed all changes yet. But I reviewed most of them and don't want to keep you waiting longer for initial feedback.

Btw, good idea to go with Astro+Starlight instead of MkDocs. 👍