[RazorDocs] Add canonical doc slugs with legacy-safe redirects

[akrock]

Summary

Add canonical browser-facing doc slugs and breadcrumb labels without breaking the existing path-based links that RazorDocs already serves.

Scope

• Generate canonical doc routes separately from raw repository paths.
• Preserve current path-based URLs as aliases or redirects.
• Normalize README-based pages and namespace pages to human-readable labels.
• Update link generation, breadcrumb generation, and canonical URL helpers to use the new routing model.
• Document how authors can override canonical slugs when needed.

Acceptance Criteria

• Existing doc URLs continue to resolve.
• Canonical routes are stable and human-readable.
• Breadcrumbs use product language instead of file names where possible.
• Tests cover legacy route resolution, canonical path generation, and breadcrumb output.