set up Github Action CI

[johnnychen94]

No description provided.

[johnnychen94]

This works for HTML now, two things left:

• generate PDF
• directly deploy to our aliyun server. To do this we might need some scp-action associated ssh-key configured.

[johnnychen94]

The deployment to our mirror server juliacn.com is provided by an action in gh-pages branch d1dc062 Every push to gh-pages would trigger that action.

I plan to merge this PR and verify if it successfully triggers the action.

The CI for pdf generator is left as future PR.

Edit: okay, the deploy action is working https://github.com/JuliaCN/JuliaZH.jl/actions/runs/1125025261