Avoid /documentation redirect when there's a root page

[finestructure]

DocC has a --enable-experimental-combined-documentation flag which can be used via .spi.yml to generate a root page. Since we direct away from /documentation to the first module's docs, we make that flag useless.

We probably want to only conditionally redirect if there's no root page. This will probably be a full feature at some point in the future.