Add product vision planning guidance

Adds guidance for PMs on expectations around product vision and how that is maintained through the year, particularly as it relates to the Product Vision 20xx label. This wasn't documented previously; the parts that are unchanged are about our year-end planning and applying the label. What's new is:

• Clarification that the vision is comprehensive from the sensing mechanisms, not a wishlist of innovative new features
• Expectation that the vision is moved forward in each release
• Modification to the kickoff doc to show a label when relevant
• Modification to kickoff meeting to prefer talking about vision items

Closes Product#197 (closed)