[DOC] Create nicer demo video / gif

[jeertmans]

What

The current example.gif was recorded a long time ago, when a lot of features did not exist, and I not quite attractive (in my opinion).

If possible, I like a small video / gif that shows, in a fews seconds, what you can do with manim-slides.

How to

Create (or modify) some example file that you will display during the example video, record yourself in such a way that you can clearly see what manim-slides is doing. A good approach to that is showing key presses with screenkey.