FEATURE REQUEST: use current repo credentials when invoked from a GitHub workflow

[UnaiUribarri-TomTom]

Currently, it is difficult to integrate sbt-ghpages in a GitHub workflow. It is not even documented how to do it and first attempts fail with authorisation errors.

It would be great if sbt-ghpages reuse the current repo credentials or detects and uses GITHUB_TOKEN environment variable, and the documentation is updated to show how to run it.