Consolidate documentation: move detailed docs to docs/ directory

Reduced markdown files from 10 to 7 for better organization.

Changes:
- Moved ARCHITECTURE.md, DEVELOPMENT.md, MIGRATION.md to docs/
- Removed tests/README.md and docs/baseline/README.md (unnecessary)
- Updated README.md with links to all documentation

Final structure (7 markdown files):
Root (3):
  - README.md (main documentation)
  - CONTRIBUTING.md (contribution guide)
  - CHANGES.md (version history)

.github/ (1):
  - PULL_REQUEST_TEMPLATE.md (GitHub template)

docs/ (3):
  - ARCHITECTURE.md (codebase structure)
  - DEVELOPMENT.md (dev setup)
  - MIGRATION.md (migration guide)

All essential information is preserved and easier to find.

Author: claude

README.md

@@ -192,7 +192,7 @@ cd python_prtree
 pip install -e ".[dev]"
 ```
 
-For detailed development setup, see [DEVELOPMENT.md](DEVELOPMENT.md).
+For detailed development setup, see [DEVELOPMENT.md](docs/DEVELOPMENT.md).
 
 ## API Reference
 
@@ -237,6 +237,14 @@ Lars Arge, Mark de Berg, Herman Haverkort, Ke Yi
 SIGMOD 2004
 [Paper](https://www.cse.ust.hk/~yike/prtree/)
 
+## Documentation
+
+- **[CONTRIBUTING.md](CONTRIBUTING.md)** - How to contribute to the project
+- **[CHANGES.md](CHANGES.md)** - Version history and changelog
+- **[docs/DEVELOPMENT.md](docs/DEVELOPMENT.md)** - Development environment setup
+- **[docs/ARCHITECTURE.md](docs/ARCHITECTURE.md)** - Codebase structure and design
+- **[docs/MIGRATION.md](docs/MIGRATION.md)** - Migration guide between versions
+
 ## License
 
 See LICENSE file for details.