Update .gitignore for generated documentation files.
Update .gitignore for generated documentation files.

google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,google/iree