Add custom version of sphinx and move breathe to doc.

.gitmodules

@@ -1,6 +1,9 @@
 [submodule "format-benchmark"]
 	path = format-benchmark
 	url = git://github.com/vitaut/format-benchmark.git
-[submodule "breathe"]
-	path = breathe
+[submodule "doc/breathe"]
+	path = doc/breathe
 	url = git://github.com/vitaut/breathe.git
+[submodule "doc/sphinx"]
+	path = doc/sphinx
+	url = git@github.com:cppformat/sphinx.git