Add doc::RustcDevGuide bootstrap step

Author: Kobzol

src/bootstrap/src/core/build_steps/doc.rs

@@ -85,6 +85,7 @@ book!(
     RustByExample, "src/doc/rust-by-example", "rust-by-example", &["ja", "zh"], submodule;
     RustdocBook, "src/doc/rustdoc", "rustdoc", &[];
     StyleGuide, "src/doc/style-guide", "style-guide", &[];
+    RustcDevGuide, "src/doc/rustc-dev-guide", "rustc-dev-guide", &[];
 );
 
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]

src/bootstrap/src/core/builder/mod.rs

@@ -994,6 +994,7 @@ impl<'a> Builder<'a> {
                 doc::EmbeddedBook,
                 doc::EditionGuide,
                 doc::StyleGuide,
+                doc::RustcDevGuide,
                 doc::Tidy,
                 doc::Bootstrap,
                 doc::Releases,