Update GitKit Slides diagrams for improved readability and better accessibility

[braughtg]

This issue is a tracking issue for the updates that need to occur to the images in the GitKit slide decks. This issue is a follow on to #89, which updates the images in the text.

Details of how to work on new images, and where and how they should be stored are given in the GitKit-Codespaces/materials/images/README.md file.

Each image or sequence of images to be updated is identified by a sub-issue below. The current unmodified slide decks can be found and downloaded from GitKit-Codespace/materials/slides.

Each of the sub-issues linked below corresponds to an image or an image sequence in the slides. To work on an image or image sequence:

1. Assign yourself to the sub-issue.3. Create a feature branch from the draw.io-images branch for your work.
2. Create the new image or image sequence as per the README.
3. Create a pull request to the draw.io-images branch instead of the main branch.