Rollup merge of #49707 - steveklabnik:rustc-book, r=QuietMisdreavus

Add "the Rustc book"

This PR introduces a new book into the documentation, "The rustc book". We already have books for Cargo, and for Rustdoc, rustc should have some too. This book is focused on *users* of rustc, and provides a nice place to write documentation for users.

I haven't put content here, but plan on scaffolding it out very soon, and wanted this PR open for a few discussions first. One of those is "what exactly should said TOC be?" I plan on having a proposed one up tomorrow, but figured I'd let people know to start thinking about it now.

The big one is that we also will want to put https://github.com/rust-lang-nursery/rustc-guide in-tree as well, and the naming is... tough. I'm proposing:

* doc.rust-lang.org/rustc is "The Rustc book", to mirror the other tools' books.
* doc.rust-lang.org/rustc-contribution is "The Rustc contribution guide", and contains that book

@nikomatsakis et al, any thoughts on this? I'm not attached to it in particular, but had to put something together to get this discussion going. I think mirroring the other tools is a good idea for this work, but am not sure where exactly that leaves yours.

Fixes https://github.com/rust-docs/team/issues/11

src/bootstrap/builder.rs

@@ -323,7 +323,7 @@ impl<'a> Builder<'a> {
                 test::Cargotest, test::Cargo, test::Rls, test::ErrorIndex, test::Distcheck,
                 test::RunMakeFullDeps,
                 test::Nomicon, test::Reference, test::RustdocBook, test::RustByExample,
-                test::TheBook, test::UnstableBook,
+                test::TheBook, test::UnstableBook, test::RustcBook,
                 test::Rustfmt, test::Miri, test::Clippy, test::RustdocJS, test::RustdocTheme,
                 // Run run-make last, since these won't pass without make on Windows
                 test::RunMake, test::RustdocUi),
@@ -331,7 +331,7 @@ impl<'a> Builder<'a> {
             Kind::Doc => describe!(doc::UnstableBook, doc::UnstableBookGen, doc::TheBook,
                 doc::Standalone, doc::Std, doc::Test, doc::WhitelistedRustc, doc::Rustc,
                 doc::ErrorIndex, doc::Nomicon, doc::Reference, doc::Rustdoc, doc::RustByExample,
-                doc::CargoBook),
+                doc::RustcBook, doc::CargoBook),
             Kind::Dist => describe!(dist::Docs, dist::RustcDocs, dist::Mingw, dist::Rustc,
                 dist::DebuggerScripts, dist::Std, dist::Analysis, dist::Src,
                 dist::PlainSourceTarball, dist::Cargo, dist::Rls, dist::Rustfmt, dist::Extended,