Freeze title of page while scrolling

[bryfry]

When scrolling through each lab, the title of the lab scrolls of the screen and sometimes we forget what lab we are actually on. It would be nice to have the title of the lab be stuck at the top (and actually remove the name of the course because hopefully you know the course you are in).

Let me know if that makes sense. @bryfry for awareness.

[bryfry]

I gave this Difficulty: ♢ because it is likely a significant departure from the css that is provided by the upstream mdBook project. Needs additional investigation to find out.