mkdocs workflow not working on some pull requests

[vincentberenz]

The bot does not get the permission to push to the gh-pages branch.
Trying to solve this by the use of a personal token.

[VincentStimper]

@vincentberenz unfortunately, this did not fix the problem.

I think the doc deployment only failed because the PR was from somebody how has not been added as a contributor to the repository. When I merged the change, it worked without an error. Hence, we can probably revert this change, since the deployment of the docs is still done with the previous version, although not at the time of the PR. The unit tests still work, which is the more important thing anyway, especially if the document is not touched. What do you think, @vincentberenz?

[vincentberenz]

@VincentStimper Agreed, even more since I removed the workflow on the pull request. I will revert the changes, and push directly to master.