Add intro to onboarding template

What does this MR do and why?

This adds an intro to the onboarding template and marks a few tasks as important. It also emphasizes that most of the tasks are more of a suggestion and appeals to the Manager of One concept.

I’m not finished working through my onboarding issue but my tentative idea is to structure it so that team-related and important tasks are closer to at the top and further reading/learning is lower. For example, the first few tasks in the Engineering Productivity Onboarding section seems more immediately relevant than the Suggested learning path one—in fact, I didn’t even realize that the former section exists. 😅