Add quick start guide for testing

Provides:
- Quick reference for common test commands
- Table of runner shortcuts
- Coverage instructions
- What's tested checklist
- Debugging tips
- Test statistics

TESTING_QUICKSTART.md

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+# Quick Test Guide
+
+Quick reference for running tests in the vgui-cicd project.
+
+## 🚀 Quick Start
+
+```bash
+# Run all tests
+python manage.py test
+
+# Or use the convenient runner
+python run_tests.py all
+```
+
+## 📋 Common Commands
+
+| Command | Description |
+|---------|-------------|
+| `python run_tests.py all` | Run all tests with verbose output |
+| `python run_tests.py cache` | Run diagram cache tests only |
+| `python run_tests.py render` | Run rendering tests only |
+| `python run_tests.py commands` | Run management command tests |
+| `python run_tests.py coverage` | Run tests with coverage report |
+| `python run_tests.py fast` | Run with fail-fast (stops at first failure) |
+
+## 🎯 Run Specific Tests
+
+```bash
+# Test a specific module
+python manage.py test diagramm_proxy
+
+# Test a specific test case
+python manage.py test abschnitte.tests.RenderTextabschnitteTestCase
+
+# Test a specific method
+python manage.py test abschnitte.tests.RenderTextabschnitteTestCase.test_render_diagram_success
+```
+
+## 📊 Coverage Report
+
+```bash
+# Generate coverage report
+python run_tests.py coverage
+
+# Or manually
+pip install coverage
+coverage run --source='.' manage.py test
+coverage report
+coverage html  # Creates htmlcov/index.html
+```
+
+## ✅ What's Tested
+
+### Diagram Caching (`diagramm_proxy/test_diagram_cache.py`)
+- Hash computation ✓
+- Cache hit/miss ✓
+- HTTP errors ✓
+- Cache clearing ✓
+- Unicode support ✓
+
+### Rendering (`abschnitte/tests.py`)
+- Markdown to HTML ✓
+- Lists (ordered/unordered) ✓
+- Tables ✓
+- Diagrams ✓
+- Code blocks ✓
+
+### Management Commands (`abschnitte/management/commands/test_clear_diagram_cache.py`)
+- Clear all cache ✓
+- Clear by type ✓
+- Help text ✓
+
+## 🐛 Debugging Failed Tests
+
+```bash
+# Run with more detail
+python manage.py test --verbosity=2
+
+# Stop at first failure
+python manage.py test --failfast
+
+# Keep test database
+python manage.py test --keepdb
+```
+
+## 📖 More Information
+
+See `Documentation/TESTING.md` for complete documentation.
+
+## 🎓 Test Statistics
+
+- **Total Test Files**: 3
+- **Total Test Cases**: 29+
+- **Lines of Test Code**: 790+
+- **Coverage Target**: >80%
+
+---
+
+**Need help?** Check the full testing guide in `Documentation/TESTING.md`