Rewrite the doc makefile for doc => src/doc

This continues to generate all documentation into doc, but it now looks for
source files in src/doc

Closes #11860
Closes #11970

.gitignore

@@ -71,20 +71,7 @@ src/.DS_Store
 /stage2/
 /stage3/
 *.bz2
-/doc/html
-/doc/latex
-/doc/std
-/doc/arena
-/doc/extra
-/doc/flate
-/doc/glob
-/doc/green
-/doc/native
-/doc/rustc
-/doc/syntax
-/doc/rustdoc
-/doc/rustuv
-/doc/rustpkg
+/doc
 /nd/
 /llvm/
 version.md