docs: refresh contributor guide

[Fhatu12]

Summary

• Refreshes CONTRIBUTING.md to match the current FastAPI, Next.js, and Tauri project structure.
• Adds contributor prerequisites, setup steps, local development workflow, and validation guidance by change type.
• Adds docs-only contribution guidance, no-secret/API-key expectations, and issue/PR etiquette.
• Notes that broad Research and Improve Product Competitiveness #187 contributions should stay small and focused.

Validation

• git diff --check HEAD~1..HEAD
• rg -n "pure Python and Tkinter|Guidlines|python app.py|Python >= 3.10|python-ci.yml|doc/" CONTRIBUTING.md || true

Scope note

This PR is intentionally focused on CONTRIBUTING.md only. It does not broaden into README wording polish, docs/index.md, or document outline / TOC functionality.

Refs #187