Rustdoc support.

Adds `m rustdoc` which generates documentation for all Rust libraries
to $OUT_DIR/soong/rustdoc.

Follow up work:

* Generate an index page that lists all modules.
* Preserve the artifacts so we can have an always-up-to-date go link.

Test: m rustdoc
Bug: None
Change-Id: Id2d6b9cbab5b02e36b575567563d7cc7606b9401

rust/Android.bp

@@ -21,6 +21,7 @@ bootstrap_go_package {
         "clippy.go",
         "compiler.go",
         "coverage.go",
+        "doc.go",
         "fuzz.go",
         "image.go",
         "library.go",