[docs] Fix SEO preview: page-specific social card render

[ahmedsadid]

What changed / motivation ?

Fixes two issues with the site's link/SEO previews.

1. Relative preview-image URL. og:image / twitter:image were served as a root-relative path (/assets/…png), which external crawlers can't always resolve.

• Added src/lib/site.ts with a shared SITE_URL (also used by the RSS feed).
• Made the image URLs absolute; added og:url, og:site_name, og:image:width/height, and *:image:alt.

2. Every page shared the homepage's card. Pages set their own <title> but inherited the layout's hardcoded og:title / og:description, so sharing any post showed the generic homepage preview.

• New <Seo> component renders per-page description / og:title / og:description / twitter:title / twitter:description.
• Removed those from the global layout (kept only invariant tags) — React does not de-duplicate meta tags, so a default + per-page tag would emit two conflicting og:titles.
• Wired <Seo> into home, docs, blog, and playground.
• Blog posts have no description frontmatter, so it's auto-derived from the post's opening paragraph(s) via src/lib/excerpt.ts.

Linked PR/Issues

Part of #1550

Additional Context

This PR builds on the blog-timestamp PR — both touch rss.ts and the blog page, so please merge that one first.

Pre-flight checklist

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

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