docs: rename Core Concepts to Reference

[BrianSeong99]

Summary

Renames the visible top-level docs section from Core Concepts to Reference while keeping the existing /core-concepts/... URL paths stable.

Changes

• Updates navbar, footer, sidebar labels, landing page copy, 404 copy, README notes, llms.txt, and skill.md references.
• Applies the same visible label update to the v0.13 and v0.14 versioned docs.
• Updates the OG image generation config and regenerated affected OG cards.

Verification

• git diff --check
• NODE_OPTIONS=--max-old-space-size=12288 npm run build after local CI-style docs aggregation
  • Build exited successfully.
  • Existing broken-link and broken-anchor warnings remain in external/versioned docs; this PR only changes labels and preserves paths.

Closes #286.