Create RetinaNet object detection guide

[sachinprasadhs]

Summary:
This Pull Request introduces comprehensive documentation detailing how to use an object detection model from Keras Hub, using RetinaNet as an example.

This tutorial includes:

• Loading and preprocessing a training dataset
• Demonstrating inference using pretrained weights
• Fine-tuning a pretrained object detector
• Custom training of an object detector
• Summary

[divyashreepathihalli]

Thanks for the PR @sachinprasadhs - since this is a guide - we should have more detailed explanations that would help users understand our framework better. Lets add more of that.

Thanks!

[mattdangerw]

Thanks! All minor comments. Also heads up I added keras-team/keras-hub#2219 for coco id - name mappings. Whenever it is released we can use it.