Post-roadmap merge updates

[willingc]

@DragaDoncila and all, Let's focus on landing this document as is. It's already way better than what we have today. It is possible in future PRs that we may be able to use a literalinclude to pull from other pages.

While I don't disagree with @melissawm and @brisvag on their points, I think the changes can come in a future PR (either soon, in 3 months, or when using the literalinclude).

Draga, I also think it is very important to add the Roadmap to the list of resources in the left-hand sidebar.

Originally posted by @willingc in #735 (comment)

Next steps after #735 merge

• Add roadmaps entry to sidebar
• Look at addressing feedback from @melissawm and @brivsag on vision and principles
• Investigate using literalinclude with myst/sphinx to add the sections for mission and principles to avoid duplication of content.