Git in-depth and backstage explained

- Why git is as it is: https://tom.preston-werner.com/2009/05/19/the-git-parable.html
- See also comments in https://www.reddit.com/r/git/s/0oW638SfcB
- https://medium.com/@jake.page91/the-guide-to-git-i-never-had-a89048d4703a
- https://get-git.readthedocs.io/en/latest/introduzione.html

Start with the Git Parable, Reddit discussion, Medium guide, and get-git documentation linked in the issue. Define the intended in-depth explanation and its location in the course before proceeding; the issue does not specify files, tests, or a completion criterion.

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