feat(seo): add semantic breadcrumbs and BreadcrumbList structured data

[KevinVandy]

Context

TanStack has a deep, meaningful content hierarchy, especially across library landing pages, documentation, blog posts, chart examples, and partner pages. These pages currently do not emit BreadcrumbList JSON-LD.

The shared src/components/Breadcrumbs.tsx component is currently a section label plus an on-page table-of-contents control. It renders div elements rather than a complete semantic breadcrumb trail, so it cannot serve as the source of truth for either accessible navigation or structured data.

Google documentation: https://developers.google.com/search/docs/appearance/structured-data/breadcrumb

Proposed scope

• Define a shared breadcrumb item model with a label and optional absolute or site-relative URL.
• Render visible breadcrumbs as nav with aria-label="Breadcrumb".
• Generate an SSR-visible BreadcrumbList JSON-LD object from the same breadcrumb items.
• Use representative user paths rather than mechanically mirroring URL segments.
• Cover at least:
  • Library landing pages: Libraries → Query
  • Docs: Libraries → Query → React Docs → Guides → Query Keys
  • Blog posts: Blog → Post title
  • Charts catalog examples: Charts → Catalog → Example title
  • Partner detail pages: Partners → Partner name
• Do not emit breadcrumb markup on the homepage, errors, or non-indexable pages.

Acceptance criteria

• Visible breadcrumbs and JSON-LD are derived from one shared data model.
• BreadcrumbList is present in the raw SSR response, not added only after hydration.
• Every BreadcrumbList contains at least two ListItem entries with contiguous positions.
• Linked breadcrumb items use absolute canonical HTTPS URLs in JSON-LD.
• The current page name matches the visible page heading/title.
• Representative routes pass Google Rich Results Test validation.
• Automated tests cover ordering, URL generation, escaping, and routes that should not emit breadcrumbs.

Notes

The current UI component can retain its on-this-page control, but that control should be separate from the hierarchical breadcrumb data.