Split licenses

[acocac]

Summary

The PR splits content and code licenses, which might be better for future readers and websites scrapping licenses of the repository.

List of changes proposed in this PR (pull-request)

• Modify the LICENSE.md to only include the license for content, in this case CC BY 4.0.
• Add a new file, LICENSE-CODE.md to indicate the MIT License for the code and notebooks, and BSD 3-Clause License for the 2i2c repo2docker

What should a reviewer concentrate their feedback on?

• [ ] Everything looks ok?

Acknowledging contributors

• [x] All contributors to this pull request are already named in the table of contributors in the README file.
• [ ] The following people should be added to the table of contributors in the README file:

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[acocac]

It looks good to me. I learned that it is recommended to add the licenses in CITATION.cff to be more machine actionable. Also add a section in the README file. I can make a PR after this one to complement what you have done. Many thanks!

@annefou thanks for the suggestion. Let's open a new PR to implement your suggestions. They sound great.