It's not easy to update submodules

[weiguoz]

We hide the submodule files on the sidebar by specifying files to _config.yaml::exclude https://github.com/sql-machine-learning/sql-machine-learning.github.io/blob/fa1aeb86267e7a8ff3cf3470104c23ad62996f71/_config.yml#L5 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.