Don't re-render documents whose contents can't change

[Rich-Harris]

This page always returns the same markup. If it were possible to determine that statically, we could render the page once and serve it from a cache thereafter.

Gets a little tricky though — even that page has a dynamic-looking expression within the nav (that determines which nav link is highlighted). Would need some form of whole-app analysis. As such, it's probably not something we can do any time soon.