Clarify the optimal usage of $sleek.setUser

Description: As a Sleekplan user, I want detailed guidance on the best way and time to use the $sleek.setUser function in order to avoid encountering errors. Problem: When attempting to use the function document.addEventListener('DOMContentLoaded', ...) to set user properties via $sleek.setUser, I encountered the error Uncaught TypeError: $sleek.setUser is not a function. The documentation seems unclear or unavailable regarding the correct implementation approach. Benefit: Clear guidelines will save developer time, prevent errors, and increase the likelihood of successful integration with Sleekplan's features.