[RazorDocs] Add evaluator-first docs landing with featured proof paths

[akrock]

Summary

Turn the default RazorDocs index into an evaluator-first landing experience that helps a reader decide whether the library or framework is worth adopting.

Scope

• Replace the generic docs index with a landing page that prioritizes Start Here, Featured Guides, Examples, and API Reference.
• Add support for curated proof cards that answer the first trust questions quickly.
• Ensure the landing layout is driven by metadata and configurable curation rather than repository folder structure.
• Keep the experience reusable in RazorDocs so Runnable is the first consumer, not a one-off customization.
• Coordinate with #101 and #107 rather than duplicating their broader IA/content work.

Acceptance Criteria

• The default docs landing highlights high-value entry points instead of directory counts.
• A site can promote a curated set of proof pages without hardcoding repo paths into the view.
• The landing experience still degrades cleanly when no curated pages are configured.
• Tests cover curated landing rendering and fallback behavior.