Document how to screen shot animations

When new animations are introduced, we need to know how to best capture new [animation samples](https://github.com/aagarwal1012/Animated-Text-Kit/tree/master/display). Specifics around:
* settings (resolution, duration, etc.)
* tooling
* optimization techniques

will be helpful.

Look at the existing animation samples in the display directory to understand the output format. Research screen recording tools for Flutter (like the Flutter driver or external tools) and settings for capturing GIFs/videos. Document the steps, recommended resolution, duration, and any optimization for file size. Test the process on one animation to verify it works.

索引モデルが issue の本文から書いたものです。