Duplicated images in the repository

[adrianboguszewski]

I found there are many duplicates in the repository e.g coco.jpg. It increases cloning time and space usage. It would be good to create a "central directory" with images and videos to use across all notebooks.

I propose:

1. Create the "data" dir in the root dir
2. Move all images and videos from specific notebooks, remove duplicates
3. Update links to media in all notebooks
4. Update contributing guide

[raymondlo84]

Let's make sure we don't make the repos any larger too.