Feat/rspress migration - #158

What's added in this PR?

• Migrates the website from the legacy Rsbuild/TanStack runtime path to Rspress SSG with generated static pages for blog/changelog detail routes.
• Preserves existing site structure and visual behavior while cleaning up legacy build/runtime artifacts (old app entry files, router tree, and Rsbuild config/deps).
• Improves metadata handling for SEO and social cards:
  • per-page title/description/canonical/OG/Twitter tags emitted at build time
  • richer social image tags (og:image:secure_url, og:image:type, twitter:image:src, etc.)
  • OG/Twitter image assets now guaranteed to exist in public output.
• Adds SITE_URL_SOCIALS (fallback: https://zephyr-cloud.io) to override thumbnail URL host only for social tags across environments.
• Fixes parity and rendering issues found during migration QA (including blog hydration mismatch and /products/ai app screenshot visibility).

What's the issue related to this PR?

The previous setup relied on client-side rendering patterns for key content pages, which weakened indexing and made social previews inconsistent. This migration targets:

• reliable static HTML for indexable content,
• consistent canonical/OG/Twitter metadata,
• and dependable social thumbnail rendering across production/preview environments.

How to test this PR?

1. Install and build:
   • pnpm install
   • pnpm build
2. Typecheck:
   • pnpm exec tsc --noEmit
3. Preview locally:
   • pnpm preview
4. Validate metadata in built HTML (example):
   • doc_build/blog/nextjs-without-lock-in-vinext-on-zephyr.html
   • confirm canonical, og:image, twitter:image, and new image meta tags are present.
5. Validate SITE_URL_SOCIALS behavior:
   • SITE_URL_SOCIALS=https://preview-social.example.com pnpm run generate:rspress-content
   • confirm generated og:image/twitter:image* use the override host,
   • confirm og:url and canonical remain https://zephyr-cloud.io/....

Checklist

• I have added explanation of the changes I made
• UI related: this PR has been visually reviewed for both web, mobile, and tablet