refactor(turing): rewrite developer guide for modern stack

- Remove all AngularJS, OpenNLP, and chatbot references
- Update frontend section to React + TypeScript + Primer CSS
- Add tech stack table and architecture diagram (Mermaid)
- Modernise dev setup: Docker Compose, React dev server with hot-reload
- Add JitPack Maven snippet for Java SDK integration
- Improve API docs with clearer tables and emoji indicators
- Add Contributing section with step-by-step workflow
- Remove admin/admin default credential references

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

Author: Alexandre Oliveira