Better Manim support

Currently, Manim videos are included as multiple video files, one per section.

This works well, but is syntactically heavier for the author, who needs a fixed-size carousel, and changing pages at the right moment.

It would be nice to:

- either have a dedicated syntax for this
- or use the full video file, and use the .json file generated in the `section` folder to know when to stop.