update _config.yml

[TRThurman]

With new layout, this file needs update.
Also note this todo (can it be done simultaneously?):

TODO: Replace by EXPRESS Language Foundation own site setup
and remove the /parent-hub directory after that.
parent_hub:
git_repo_url: https://github.com/expresslang/ap210.org
git_repo_branch: main
home_url: https://www.ap210.org/

[ronaldtse]

@TRThurman this is not exactly easy but we should still do it.