Add lean week (Oct 2025) content

[unnawut]

Not sure how best to list these down in a nice way. Probably need a bit of thinking on the layout.

https://www.youtube.com/playlist?list=PLJqWcTqh_zKGPctGzVOBllZnCPQj-g7d3

Plus all the related content here: https://github.com/leanEthereum/pm/blob/main/workshops-and-interops/2025/lean-week-cambridge/index.md