Add an info tooltip to dashboard plots

We expose many diagnostic dashboard plots to users. This is great as it provides a detailed, real-time look into a cluster's state.

However, it may not be immediately clear to users (especially new users) what information is actually being displayed in each plot. For example, users may not know that the central circle in the "cluster map" plot (a.k.a the "pew-pew plot") represents the scheduler while the surrounding circles represent workers.

One way we could better inform users would be to add an info icon ℹ️ to our dashboard plots which, when users hover their mouse over it, displays information about what the plot is displaying.

cc @charlesbluca who has been thinking about dashboard visualizations recently