docs: remove contributing guide from mdBook

Contributing info now lives in README and issue templates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: GiggleLiu

docs/src/SUMMARY.md

@@ -11,5 +11,4 @@
 # Developer Guide
 
 - [API Reference](./api.md)
-- [Contributing](./contributing.md)
 - [CLAUDE.md](./claude.md)