Convert /staging/[[...staging]].jsx to App Router

[genehack]

This is part of the Convert static-site from Pages Router to App Router work. See there for additional context.

This page uses essentially the same set of components as the /pathogens page. Once #1066 is completed, this should be a small amount of additional work, moving this page into /static-site/app/, pointing it to the new versions of components added during the /pathogens conversion, and updating the dynamic routing code. (We should be able to render all versions of this server-side, I believe?)