Constant rustdoc::clean::utils::DOC_RUST_LANG_ORG_CHANNEL

pub(crate) const DOC_RUST_LANG_ORG_CHANNEL: &str = env!("DOC_RUST_LANG_ORG_CHANNEL");

A link to doc.rust-lang.org that includes the channel name. Use this instead of manual links so that the channel is consistent.

Set by bootstrap::Builder::doc_rust_lang_org_channel in order to keep tests passing on beta/stable.