rustdoc: Add unstable option to only emit shared/crate-specific files
The intended use case is for docs.rs, which can now copy exactly the
files it cares about, rather than having to guess based on whether they
have a resource suffix or not. In particular, some files have a resource
suffix but cannot be shared between crates: https://github.com/rust-lang/docs.rs/pull/1312#issuecomment-798783688
The end goal is to fixrust-lang/docs.rs#1327 by reverting rust-lang/docs.rs#1324.
This obsoletes `--print=unversioned-files`, which I plan to remove as
soon as docs.rs stops using it.
I recommend reviewing this one commit at a time.
r? ``@GuillaumeGomez`` cc ``@Nemo157`` ``@pietroalbini``
Remove theme.js file
Fixes#82616.
The first commit moves the `theme.js` file into `main.js`, which requires to also run a small `.replace` on the `main.js` content.
The second commit is just a small cleanup to centralize DOM ids.
Since it removes a file from rustdoc output: cc `@rust-lang/docs-rs`
cc `@jsha`
r? `@jyn514`
