seo: add JSON-LD structured data to docs pages

[planetf1]

Background

No <script type="application/ld+json"> blocks are present on any page. Adding structured data improves rich-result eligibility in Google Search.

Suggested schema types

• BreadcrumbList — on all docs pages; Docusaurus has a plugin or this can be added via a swizzled theme component
• TechArticle — on individual doc pages, using the page title and description frontmatter
• Organization — on the home/entry page for the IBM Granite brand

References

• Google structured data docs
• Docusaurus — custom head metadata

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