Auto merge of #43863 - steveklabnik:ship-the-rustdoc-book, r=frewsxcv

Ship the rustdoc book

Fixes #42322, as it's the last step.

Blocked on https://github.com/rust-lang/rust/pull/43790, though they will not conflict.

r? @rust-lang/docs

src/bootstrap/builder.rs

@@ -257,7 +257,7 @@ impl<'a> Builder<'a> {
             Kind::Bench => describe!(check::Crate, check::CrateLibrustc),
             Kind::Doc => describe!(doc::UnstableBook, doc::UnstableBookGen, doc::TheBook,
                 doc::Standalone, doc::Std, doc::Test, doc::Rustc, doc::ErrorIndex, doc::Nomicon,
-                doc::Reference),
+                doc::Reference, doc::Rustdoc),
             Kind::Dist => describe!(dist::Docs, dist::Mingw, dist::Rustc, dist::DebuggerScripts,
                 dist::Std, dist::Analysis, dist::Src, dist::PlainSourceTarball, dist::Cargo,
                 dist::Rls, dist::Extended, dist::HashSign),