Simplify/improve GitHub actions YAML setting example

[tkf]

This PR contains three independent patches:

• Simplify the workflow YAML by not using strategy.matrix. IIUC you don't need this if you are only running one combination of julia and OS. Ref: Simplify CompatHelper.yml example in README.md JuliaRegistries/CompatHelper.jl#171
• Update to actions/checkout v2: https://github.com/actions/checkout/releases/tag/v2.0.0. I didn't find any breaking change: Build documentation via GitHub Actions tkf/MyPlayground.jl#21
• Change on setting so that push_preview = true works out-of-the-box.

[fredrikekre]

Thanks. I believe the matrix strategy was the only working one back in GitHub actions beta when I first tried this.

[fredrikekre]

Suggested change

	pull_request:
	pull_request: # only needed for preview builds (push_preview=true in deploydocs)

[tkf]

Isn't this also required for running documenter in PRs from forks? (My post in the OP was a bit misleading.)

I guess people familiar with Travis would find this setup natural.

[fredrikekre]

Ah, right.