Add `-` (stdin) support in rustdoc This PR adds support for the special `-` input which threats the input as coming from *stdin* instead of being a filepath. Doing this also makes `rustdoc` consistent with `rustc` and ~~every~~ other tools. Full [motivation](https://github.com/rust-lang/rust/pull/124611#issuecomment-2094234876). Fixes https://github.com/rust-lang/rust/issues/123671 r? `@fmease`