Add beginner-friendly explanatory comments (educational use case)

Hi 👋,

I used Learning mesh-based simulation with Graph Networks code as part of my scientific machine learning final project, and while working through it, I added some inline comments for myself to better understand the flow, assumptions, and math behind the implementation.

These comments don’t change functionality — they’re purely explanatory — but they helped me significantly in building intuition, and I believe they could be useful for other beginners or students using this code for learning.

This isn’t addressing a listed issue, so I wanted to ask first: would you be open to a PR that adds optional beginner-friendly comments?

Happy to:
• keep comments concise
• follow the project’s style
• limit changes strictly to documentation/comments

Thanks for maintaining this project!