docs(site): add Open Graph, Twitter card and JSON-LD structured data

[astra-intelligence]

What

Implements the Social previews and structured data section of #17 (overrides/main.html, extrahead) and the C-library epic's approach (libphash#51, libphash#52):

• overrides/main.html — a Zensical extrahead block (wired via project.theme.custom_dir) that emits, on every page:
  • og:title, og:description, og:url (the canonical URL), og:type, og:site_name, og:image with width/height/alt
  • twitter:card = summary_large_image plus twitter:title/description/image
  • JSON-LD SoftwareSourceCode on the front page (name, description, programmingLanguage: Python, codeRepository, license, PyPI url, isBasedOn libphash's repo + site) and JSON-LD TechArticle on the guide pages
• docs/assets/social-preview.png — a 1200×630 preview generated from the site's own hash-bit logo grid in its accent #2a78d6, regenerable via the committed Pillow-only script tools/site/make-social-preview.py (no machine paths).
• zensical.toml — custom_dir = "overrides".

Verification

• uv run --locked --only-group docs zensical build --strict passes clean.
• All 25 built pages emit og:image + twitter:card with absolute URLs (https://gudoshnikovn.github.io/python-libphash/assets/social-preview.png).
• Front page carries SoftwareSourceCode JSON-LD; guide pages carry TechArticle JSON-LD; both validated as parseable JSON.
• Preview card vision-QA'd: text fully visible, no clipping, on-brand.

Checklist addressed

From #17's "Social previews and structured data (overrides/main.html, extrahead)" section:

• og:* + twitter:card=summary_large_image with canonical og:url and an absolute image URL
• 1200×630 preview image generated in the repository (tools/site/make-social-preview.py + tracked PNG)
• JSON-LD SoftwareSourceCode on the front page, TechArticle on guide pages

The per-page title/description front matter and the duplicate-description strict check are the remaining part of #17 and can follow in a separate PR (they touch every page's content rather than the theme).

[astra-intelligence]

Upstream has since implemented this exact feature in commit aff808d (docs(site): page titles, descriptions, social previews and structured data, Oct 16) — per-page title/description, the full OG/Twitter set, JSON-LD (WebSite, SoftwareSourceCode, TechArticle, BreadcrumbList), a 1200x630 social.png, and noindex on the 404. That implementation is more complete than this PR, so this PR is superseded. Closing it to keep the queue clean — no action needed from you. Thanks for the great repo!