Add worked example with mAP optimisation to docs

[robmarkcole]

Good example writeups:

• https://github.com/shayanalibhatti/Retail-Store-Item-Detection-using-YOLOv5
• https://blog.roboflow.com/using-computer-vision-to-count-fish-populations/
• https://blog.roboflow.com/computer-vision-american-sign-language/