Add chapter on Git internals

[lnnrtwttkhn]

Hi @konradpa, if you feel like adding new content, I think it would be nice adding content about git internals / Inside the .git folder, either adding to existing chapters or adding a new one.

Some resources:

• [ ] The chapter "pro git" by chacon & straub is probably a good starting point: https://git-scm.com/book/en/v2

More resources I came across:

• [ ] https://maryrosecook.com/blog/post/git-from-the-inside-out
• [ ] https://wyag.thb.lt
• [ ] https://jvns.ca/blog/2024/01/26/inside-git/
• [ ] https://maryrosecook.com/blog/post/git-in-six-hundred-words

I'm not sure about the best structure though: as a separate chapter or as "Find out more" boxes in the existing chapters.