rustdoc: refactor: move all static-file include!s into a single module
This is a smaller refactor that creates a new module `rustdoc::html::static_files`, which contains a bunch of `static` variables with all the files in `html/static` that we use. The idea behind moving them all here was to remove the duplicate `include_bytes!()` that are used by the theme-checker code. It also continues to centralize more operations in rustdoc.