add external animations as pre-requisites as motivational reading

[avallecam]

• https://www.nytimes.com/interactive/2025/04/12/upshot/measles-herd-immunity-explainer.html
• https://www.nytimes.com/video/upshot/100000010076565/why-is-it-so-hard-to-stop-a-measles-outbreak.html