Remove Sonar cookbook content and restore docs sync

[andrewmadson-pplx]

Why

The public cookbook is intended to feed the live Mintlify cookbook, but that connection stopped succeeding after December 17, 2025. The workflow used tag-based Actions rejected by the organization's full-SHA pinning policy and still copied into the obsolete top-level cookbook/ path instead of docs/cookbook/. The two repositories consequently diverged.

At the same time, public Sonar support ends September 27, 2026, so Sonar-dependent cookbook content and the unmaintained community showcase need to leave the published surface without losing the useful ideas worth rebuilding.

What changed

• Makes this repository's docs/ and static/ content match the cleaned Mintlify cookbook source: 27 pages and 24 Agent API, Search API, or Embeddings recipes.
• Removes all 26 community showcase pages and 13 Sonar-using examples/guides while preserving unrelated content.
• Adds SONAR_AGENT_MIGRATION_BACKLOG.md, linked to the migration project, with pinned source snapshots, a complete removal inventory, prioritized Agent API rebuild candidates, proposed replacements, and acceptance criteria.
• Repairs the docs sync workflow with full-SHA Action pins, the correct docs/cookbook/ destination, deletion-aware syncing, Node 20, and cookbook navigation/gallery regeneration.
• Updates contributor guidance and PR validation for the current examples-and-guides structure.

Deleted material remains available from the pinned commits recorded in the migration backlog.

Impact and merge order

Merge the companion ppl-ai/api-docs cleanup PR first. Once this PR merges, the repaired workflow will sync this repository into the live Mintlify source on ppl-ai/api-docs main.