Allow Plain and Blog apps to have documents/files anywhere

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What we really want is for all apps to automatically turn all their .markdown files into Statocles::Page::Document objects, turn all other files into Statocles::Page::File objects, and then for some apps to additionally create more pages based on the document pages they found.

This should happen in the Statocles::App role. Then, the Blog app can add its own pages, the Plain app may not need to do anything, the Static app needs to consider everything a file, and the Perldoc app doesn't know what a document is at all.

This is surprisingly not a breaking change, I don't think...

[preaction]

This is mostly an idea because things like #339 need to be applied to all apps, really.

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This is only relevant for apps that use Statocles::Store, so it should probably Statocles::App::Role::Store.

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We didn't allow blogs to really have things anywhere yet, because of having special "drafts" folders. We might instead in the future create a type of visible-but-ignored directory prefix, like _ for example.