Rollup merge of rust-lang#40222 - steveklabnik:extract-nomicon, r=alexcrichton

Extract nomicon to its own repo

part of rust-lang#39588

same as rust-lang#40213 but for the nomicon

r? @alexcrichton

Author: GuillaumeGomez

.gitmodules

@@ -18,6 +18,9 @@
 [submodule "src/liblibc"]
 	path = src/liblibc
 	url = https://github.com/rust-lang/libc.git
+[submodule "src/doc/nomicon"]
+	path = src/doc/nomicon
+	url = https://github.com/rust-lang-nursery/nomicon
 [submodule "src/tools/cargo"]
 	path = src/tools/cargo
 	url = https://github.com/rust-lang/cargo