Support using ssh-agent, recommend using webfactory/ssh-agent actions

[mortenpi]

Sort of follow-up to #2018, but I came across the webfactory/ssh-agent action. It should be possible to suggest that people add this to their docs workflows

- uses: webfactory/ssh-agent@v0.7.0
  with:
    ssh-private-key: ${{ secrets.DOCUMENTER_KEY }} # can't be base64 encoded though I assume

And then in deploydocs we could then just authenticate with the agent. This way we could avoid materializing DOCUMENTER_KEY on disk, and could actually avoid exposing the secret to Documenter altogether.

Would need some work to make work, and I think we'd also have to indicate to deploydocs that it should try the agent (something like deploydocs(auth=:sshagent))?

[odow]

Related #1889