update list of ET related lectures

[Roland Haas]

The list of NR lectures on https://docs.einsteintoolkit.org/et-docs/Main_Page#Documentation should be updated to include lectures from the last couple of ET summer schools (KCL, LSU and NCSA at least) that have recorded lectures.

Sources for lectures:

• https://sites.google.com/view/eetm2019/home
• https://youtube.com/playlist?list=PLzB9aSNzlOE-gKDpYA9v6YvNnTKJqCh_G
• https://einsteintoolkit.github.io/et2021uiuc/
• https://icerm.brown.edu/video_archive/?play=2356

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[Roland Haas]

This should be easily doable as a hackathon item (but might not be interesting).