Update workflows for githubactions + documenter + tagbot

[ericphanson]

One needs a few extra steps to have GithubActions CI build docs hosted on GitHub, and to also have TagBot trigger stable docs builds:

• For the first, one needs to add an ssh key for documenter docs to deploy from github actions (i.e. GitHub Pages build not triggered with GITHUB_TOKEN JuliaDocs/Documenter.jl#1177). Example: https://github.com/ericphanson/UnbalancedOptimalTransport.jl/blob/8585054cccb74a05a4624f1f915b9600f036eab2/.github/workflows/ci.yml#L50
• For the second, one needs to add an ssh key to tagbot so it can trigger the stable docs build (Support SSH keys for pushing tags JuliaRegistries/TagBot#49). Example: https://github.com/ericphanson/UnbalancedOptimalTransport.jl/blob/8585054cccb74a05a4624f1f915b9600f036eab2/.github/workflows/TagBot.yml#L12
• For some reason, once the latter is added, one then runs into an unknown host issue for the docs build, so needs to add a keyscan step (Support SSH keys for pushing tags JuliaRegistries/TagBot#49 (comment)). The strange bit is that this issue didn't arise without tagbot in the picture. Example: https://github.com/ericphanson/UnbalancedOptimalTransport.jl/blob/8585054cccb74a05a4624f1f915b9600f036eab2/.github/workflows/ci.yml#L40-L42

It would be great if PkgTemplates could incorporate these modifications so that github actions + documenter + tagbot all works together without needing extra steps. (Of course, the user will still have to add the key as a secret by hand, like in the case for travis).

[christopher-dG]

Thanks for reporting, I'll add the new plugin options to the plugin soon.

For the third one, I still think this should go into Documenter but I can look into adding it as a separate step here too. I've noticed that it doesn't happen if you run a shell script rather than a Docker container, so presumably the shell environment has its hosts sorted out for GitHub already.

[ericphanson]

Thanks! (And thanks for all your help over on the tagbot issue). I'll ask on the Documenter issue JuliaDocs/Documenter.jl#625 about whether it should be handled in Documenter proper, or in the github actions CI file.