Reworked IA of the UX Research handbook
Currently, UX Research content is all on one page in the handbook. This approach no longer scales, as the page contains too much content to be readable.
This MR breaks the content into a new Information Architecture with two pages: UX Research (high-level info for anyone) and UX Researcher Workflow (content that's specific to the UX Research team). It also moves some content to other pages and removes content about MailChimp.