Per-page social preview cards for docs and blog

[jkelleyrtp]

Summary

Links to docs pages and blog posts currently unfurl as the generic homepage card (same <title>, description and opengraph.png on every route). This PR makes every page carry its own metadata and gives docs/blog pages a generated 1200×630 preview image.

Per-route meta (packages/docsite/src/seo.rs) — Head is now driven by PageMeta::for_route(&route):

• <title>, meta[description], link[canonical], full og:* set (og:url, og:image + width/height/alt, og:site_name, og:type website|article), twitter:* (summary_large_image, @dioxuslabs), and for blog posts article:published_time / article:author.
• Docs: title "{page} – Dioxus 0.7 Docs"; generic pages ("Overview") get their chapter or SUMMARY part prefixed ("Core Concepts: Overview – …"). Description = first paragraph of the page's markdown (markdown_description, via pulldown-cmark: skips headings/html/code/{{#include}}, ~200 chars).
• Blog: title/description/date/category come from the existing "Title $ Category $ Date $ Description" SUMMARY format (page_to_meta is now shared and no longer panics on malformed lines). Also fixed introducing-dioxus.md's swapped category/date in SUMMARY.md, which was showing "Release Notes" as the date on the blog list.
• Old docs versions keep noindex and fall back to the default image; the default image now goes through asset!() so it resolves to the real hashed path (the old hardcoded /assets/static/opengraph.png URL was likely stale).

Generated OG images (packages/docsite/src/og_image.rs, server feature only)

• Runs as part of the existing server --generate-search-index step in gh-pages.yml, so no workflow changes: writes public/og/<slug>.png for all 0.7 docs pages, blog posts, /blog, /awesome, /deploy (109 images, ~0.1 s each in release).
• Renders an SVG template with resvg (bundled Inter fonts under packages/docsite/og/fonts, OFL) — dark card with the DNA logo, breadcrumb eyebrow (DOCS · 0.7 · CORE CONCEPTS · BUILDING USER INTERFACES), title (auto-shrinks 76→52px, ≤3 lines), description (≤2 lines, ellipsised). Text is measured with ab_glyph for wrapping; inner breadcrumbs are dropped when the eyebrow doesn't fit.
• seo::og_image_slug(route) (/learn/0.7/essentials/ui/rsx → learn-0.7-essentials-ui-rsx) is the single source for both the og:image URL and the file name.

Verified locally with the same commands as CI (dx build --web --fullstack true --features fullstack,production --release --ssg then server --generate-search-index): SSG HTML for /, /learn/0.7/essentials/ui/rsx, /blog/release-070 contains each tag exactly once with the expected values, and public/og/ contains 109 PNGs.

To preview cards locally without a full build: OG_PREVIEW_DIR=/tmp/og cargo test -p dioxus_docs_site --features server og_image.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/c3136db61b114f4bba89e1d74f9ed07c
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/c3136db61b114f4bba89e1d74f9ed07c?variant=devin-insiders
Requested by: @jkelleyrtp

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