It's not easy to update submodules

[weiguoz]

We hide the submodule files on the sidebar by specifying files to _config.yaml::exclude

sql-machine-learning.github.io/_config.yml

Line 5 in fa1aeb8

exclude: ['*.sql', '*.go', '.gitignore', '*.sh', '*.proto', '*.y', '*.py', 'vendor', 'gohive/doc/design.md', 'gohive/docker/README.md', 'pysqlflow/doc/design/client_design_doc.md', 'pysqlflow/doc/design/magic_design_doc.md', 'sqlflow/doc/mysql-setup.md', 'sqlflow/doc/tutorial/aspara2019/', 'sqlflow/doc/design/design_ant_xgboost.md', 'sqlflow/doc/design/design_workflow.md', 'sqlflow/python/README.md', 'sqlflow/parser/README.md', 'sqlflow/parser/remote/README.md', 'sqlflow/parser/remote/hiveql/README.md', 'sqlflow/scripts/markdown_link_check/README.md', 'sqlflow/pkg/sql/codegen/xgboost/README.md', 'sqlflow/doc/model_parameter.md']

As the submodule keep updating, we should fix the _config.yaml::exclude to exclude the new files.
I think it's inconvenient and we should find a better way.