Add a proto_library target for stardoc_output.proto.

[phst]

This makes it possible to use the protocol buffer definitions in Bazel-built binaries generating or consuming output messages.

[tetromino]

Can I ask what is the intended usage?

Note that there is already a proto_library target for this proto in the bazel source tree - see @io_bazel//src/main/java/com/google/devtools/build/skydoc/rendering/proto - which you might be able to use.

[tetromino]

Fixed by #162