Improve JSON-LD structured data

[albinazs]

Fixes #487

Description

Reported in #485 ("JSON-LD structured data - Required", "JSON-LD entity linking (sameAs) - Recommended")

Replaces the hand-built WebPage JSON-LD with one linked @graph on every page:

• Organization (Wagtail): logo, plus sameAs links to the profiles wagtail.org links to.
• WebSite (the guide): name, description, published by Wagtail.
• SoftwareSourceCode (Wagtail CMS): sameAs links to GitHub, Wikipedia and Wikidata so search engines and AI tools can tell the CMS apart from the bird. It isn't a SoftwareApplication because Google would flag that as a broken rich result without ratings or reviews.
• WebPage (homepage) or TechArticle (content pages): headline, description, URL, language, dates.
• BreadcrumbList on content pages, built from published ancestors only.

Tested with https://validator.schema.org/ and https://search.google.com/test/rich-results

AI usage

AI-assisted issue investigation, code generation, and PR description.

[thibaudcolas]

I had to merge manually - thank you!