Rollup merge of rust-lang#56373 - steveklabnik:update-books, r=Mark-Simulacrum

Update books

This pulls in @gankro 's final nomicon changes, and adds the edition guide as a submodule.

This is the final doc backport to beta for 1.31, as well.

Author: kennytm

.gitmodules

@@ -65,3 +65,6 @@
 [submodule "src/doc/rustc-guide"]
 	path = src/doc/rustc-guide
 	url = https://github.com/rust-lang/rustc-guide.git
+[submodule "src/doc/edition-guide"]
+	path = src/doc/edition-guide
+	url = https://github.com/rust-lang-nursery/edition-guide

src/bootstrap/doc.rs

@@ -70,6 +70,7 @@ macro_rules! book {
 book!(
     Nomicon, "src/doc/nomicon", "nomicon";
     Reference, "src/doc/reference", "reference";
+    EditionGuide, "src/doc/edition-guide", "edition-guide";
     RustdocBook, "src/doc/rustdoc", "rustdoc";
     RustcBook, "src/doc/rustc", "rustc";
     RustByExample, "src/doc/rust-by-example", "rust-by-example";