Use `@path` more selectively

FYI, this will `read` all files under `pkgdir(PlutoStaticHTML)` into the struct stored in `PKGDIR`, so might make the load time slower if you happen to have big files in there. Best to limit what goes in `@path` based only on the folders that you'll actually need at runtime.

_Originally posted by @MichaelHatherly in https://github.com/rikhuijzer/PlutoStaticHTML.jl/pull/124#discussion_r942248989_