MegaDetector notebook: download dataset and fine-tune section, .gitignore (#71)

- Add section 3b: download COCO128, remap to animal/person/vehicle, prepare data.yaml
- Add YOLOv5 training from MegaDetector weights (clone repo + train.py)
- Add optional cell to load fine-tuned best.pt for inference
- Ignore examples/notebooks/data/, runs/, yolov5/

Author: nee1k

.gitignore

@@ -1,4 +1,7 @@
 .idea/
 .venv/
 __pycache__/
-*.egg-info
+*.egg-info
+examples/notebooks/data/
+examples/notebooks/runs/
+examples/notebooks/yolov5/