docs: add meta tags and SEO how-to

[aryansk]

Summary

• promote the hidden metadata stub into a findable How To guide for meta tags and SEO
• show the React 19 approach for dynamic titles, descriptions, and canonical URLs
• document the meta and links exports, JSON-LD, and parent/child route behavior
• explain why data-dependent links belong in a route component rather than links

Closes #12821

Validation

• git diff --check
• checked the examples against the current route-module and root-route documentation
• full documentation build not run locally because this sparse checkout does not include installed documentation dependencies

AI disclosure

This draft was prepared with AI assistance. No claim of human review is made; it remains a draft for maintainer review.