Growth/Analytics updates/Personalized first day experiment plan

Our Personalized first day intervention will deploy a short survey to users who register on our target wikis. In preparation for that going live, we are considering whether to run any experiments with that survey, and if we do, what type of experiments to run. We started out by discussing this in T206380, but then turned it into a larger plan that doesn't fit Phabricator well. This page describes and explains our current plans. Feel free to follow up on the |talk page with any questions or comments you might have, this is a work in progress and we welcome feedback!