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
Dan Albert