Assume branch *is* from fork when not possible to verify.

[fredrikekre]

This patch changes the default assumption when verifying if a pull request is coming from a branch or not from true to false.

Fixes #1961, fixes #1965, closes #1966.

[fredrikekre]

https://github.com/JuliaDocs/Documenter.jl/pull/1969/files?w=1 for review.

[mortenpi]

LGTM. Also, nice job sleuthing the GR stuff.

1. I think this could use a brief CHANGELOG note, since it slightly changes behavior.

2. Could we also add a small note to the docts, something like

   On GitHub Actions, GITHUB_TOKEN must be present for previews to work, even if DOCUMENTER_KEY is being used to deploy."

   to

Documenter.jl/src/Documenter.jl

Lines 474 to 478 in 2d02013

	!!! note
	Pull requests made from forks will not have previews.
	Hosting previews requires access to the deploy key.
	Therefore, previews are available only for pull requests that were
	submitted directly from the main repository.