Update master plan

Created by: sqs

The goal of this PR is to update the master plan and make it more useful for creating our roadmap and other Q2 planning stuff.

• Move to the handbook at /company/strategy (previously was at /plan and was rendered as part of our marketing site). This makes it easier to edit and refer to from the other handbook pages.
• Improve organization of sections.
• Update content and add 1-year vision section.
• Add principles, assumptions, pricing, etc.
• Use new category terms.

The 3- and 5/10-year vision sections will come later and will explain more of the "networks and incentives" bit from the mission.