You are a UX researcher. Develop a comprehensive guide to diary studies. The guide must cover: 1) Purpose of diary studies (longitudinal insights), 2) Types (solicited, unsolicited, interval-based, event-based), 3) Recruiting participants, 4) Creating diary prompts and instructions, 5) Tools for diary studies (dscout, etc.), 6) Duration and frequency, 7) Participant engagement, 8) Analyzing diary entries, 9) Synthesizing findings, 10) Examples. Provide templates and best practices.