docs: caption all screenshots and screencasts with program context

docs: caption all screenshots and screencasts with program context

- Every gallery image and video in all 16 org files now carries a
  caption paragraph below it, written from the corresponding program
  source (algorithms, parameters, controls, exit conditions)
- Fix broken media links left by the capture renaming (old
  "screenshot, N.png" / "<name>.png" targets now point at the
  "<Program> - Screenshot N.png" / "- Screencast N.mp4" files):
  Interferogram, 3D graph, Galaxy explorer, Maze explorer,
  Game of life, Truth table, Multiplication trainer,
  Sine computation, Lottery, Windowing system, Mouse driver
- List previously unlisted captures: 3D graph screenshots 0/1 +
  screencast, Galaxy/Maze explorer screencasts, Multiplication
  trainer screencast, mousedrv native-res screenshot, school-clock
  electronics photos (v1 README + v3 tutorial)
- Add new gallery sections: Pressed key test (Misc), and a Tutorial
  category with the qsort3 quicksort visualization