This PR adds events to track engagement in our docs. Since we use a
scrollable `<div>` in our docs, the normal `scroll` events in GA do not
work.
A new `<ScrollableContent>` component is added that will do two things:
- Send `scroll_25`, `scroll_50`, `scroll_75`, and `scroll_90` events
whenever the user scrolls to 25%, 50%, 75%, of 90% of the content
- Optionally reset scroll top to zero whenever router changes (existing
behavior)
All of the places where we have content in a scrollable `<div>` is
replaced with `<ScrollableContent>`.
Note: 90% means user has reached the bottom, since it's not usually
possible to get to 100%.
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