command-line-video-demo: add the video demo

Create this documentation page to have a stable URL in case we need to
swap out the YouTube video in the future.

docs/command-line-video-demo.md

@@ -0,0 +1,10 @@
+---
+sidebar_position: 3
+title: Video Demo
+---
+
+# Divvi Up Command Line Video Demo
+
+The video below shows the [command line tutorial](./command-line-tutorial) with some additional explanation. If you prefer to try it for yourself on your computer checkout the [command line tutorial](./command-line-tutorial).
+
+<iframe width="840" height="473" src="https://www.youtube.com/embed/sCJlJAy1Fyg?si=MUUburZe1XbOjvq2" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>