Adding timeline

[trevmoy]

Adding for 3rd explore badge. The timeline gives an easy to understand overview of the entire course using mermaid syntax.

[trevmoy]

Sorry, @brownsarahm for closing and reopening I didn't mean to. Everything should be formatted correctly.

[brownsarahm]

this won't be visible on the site unless it's added to a toc.

we should also try to render it vertically and from the same csv that generates the schedule tables. In order to use it for next semester, we would need those two changes.