Rustdoc: Add unstable --no-html-source flag
Fixes https://github.com/rust-lang/rust/issues/115060.
This is the equivalent of `#![doc(no_html_source)]` but on the command-line. It disables the generation of the source pages (and of the links pointing to them as well).
The motivation behind this is to enable to reduce documentation size when generating it in some locations without enforcing this to end users or adding a new feature to enable/disable the crate attribute.
r? `@notriddle`
5cbc00fb7e