Move index to app router

[jonathanawesome]

This PR attempts migrates the site index from pages router to app router and sets up the proper layout files.

Notes:

• Shuffled layouts around such that the root layout provides core html/head/body elements. (main) and (conf)/conf/year implement viz-only.
• not-found.tsx uses NavBar and Footer because it needs to live at the /app root and can't use our (main) layout.
• not-found.tsx uses a hacky way to get <title> working. We can remove this after we've updated to modern nextjs.

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