Check for empty CI environment variables

[knuesel]

This PR changes deployconfig.jl to check that DOCUMENTER_KEY, GITHUB_TOKEN and GITHUB_ACTOR are non-empty. Currently it only checks if they are defined.

Why the current behavior is a problem: Many (most?) Julia packages configure CI with the following in the docs job:

        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }}

(That's what's generated by PkgTemplates and suggested by the Documenter manual.)

This means that deploys don't work with the initial configuration: both variables are defined, but DOCUMENTER_KEY will be empty unless/until the user configures the secret. At deploy time, Documenter gives priority to DOCUMENTER_KEY so the deploy will fail.

With this PR, Documenter will fall back to GITHUB_TOKEN when DOCUMENTER_KEY is empty, so deploys work "out of the box" since the token is always defined.

The PR also includes small docs change:

• Mention that push_review doesn't work from forks
• Make it clearer that GITHUB_TOKEN and DOCUMENTER_KEY are not necessarily both needed

[knuesel]

I just force-pushed a small change: I was using a bad test for empty string (!==, not reliable).

[mortenpi]

Also LGTM. And thanks a lot for for the detailed rationale in the OP!