Add analytics to urbit.org buttons

[thelifeandtimes]

Umami allows for us to track analytics on button clicks. We should implement this in order to better understand how people traverse the site flows, particularly around the 'getting started' experience. We want to do this in the most minimally invasive way, as specific user data is unnecessary for improving the site experience. The event tracking functionality in Umami is likely the best way to do this: https://umami.is/docs/track-events