Clarify Licensing of Teaching Materials (e.g., Tutorials)

[namiroues]

As suggested by @jennaswa in PR comment, we should add a clear statement regarding the licensing of teaching and training materials (e.g., tutorials, workshop notebooks).

These is currently no public statement about the openness of the teaching content. Most of these are likely under Creative Commons (CC-BY) licenses, but we need to confirm this.

Before making this update, we should:

• Review and verify the licenses associated with all existing tutorials and educational content (Jupyter notebooks, website tutorials, workshop materials).
• Confirm whether CC-BY 4.0 is appropriate and consistent across all materials.
• Draft a short paragraph to add under the "Open source" or a new "Teaching & Learning" section on the website, stating MDAnalysis's commitment to open educational resources.