First draft text of proposed video clips

[david-a-wheeler]

This creates simple text to introduce major sections with short video clips. I intend for one to be done by CRob.

One feedback item we got was that many people wanted a little more media (video) to break the monotony. The problem is that creating videos takes a lot of time, and more importantly, makes editing the material hard. We might be able to resolve that with AI, but that's a separate discussion. The proposed solution is to have short video clips that introduce chapters - they are unlikely to need updating, yet they'll help break up the material.

[david-a-wheeler]

I've seen no objections, and the originals have been around for a while. I'm going to merge in this text; if we want further changes, we can create new PRs.