Add a project license

[Lling0000]

The repository is public and intended to be shared, but it currently does not specify a license.

A clear license would help readers understand whether and how they can reuse, translate, quote, or adapt the guide.

Possible options to consider:

• MIT, if the owner wants a simple permissive license for code and docs.
• CC BY 4.0, if the guide should be treated mainly as written content with attribution.
• CC BY-SA 4.0, if adaptations should remain share-alike.

This issue needs the repository owner to choose the intended license before a PR is opened.