Automate ts/scss compilation

[yufeih]

Automate docfx.js, docfx.css compilation either using git pre-commit hook, or a standalone GitHub action.

[yufeih]

Doesn't seem possible with GH action, but at the same time we don't want to review other's dist code.

https://github.community/t/how-to-push-to-protected-branches-in-a-github-action/16101/55?page=3
community/community#13836