docs: add an example about fix mode on GitHub Actions

[ferrarimarco]

Readiness checklist

In order to have this pull request merged, complete the following tasks.

Pull request author tasks

• I checked that all workflows return a success.
• I included all the needed documentation for this change.
• I provided the necessary tests.
• I squashed all the commits into a single commit.
• I followed the Conventional Commit v1.0.0 spec.
• I wrote the necessary upgrade instructions in the upgrade guide.
• If this pull request is about and existing issue,
  I added the Fix #ISSUE_NUMBER or Close #ISSUE_NUMBER text to the description of the pull request.

Super-linter maintainer tasks

• Label as breaking if this change breaks compatibility with the previous released version.
• Label as either: automation, bug, documentation, enhancement, infrastructure.
• Add the pull request to a milestone, eventually creating one, that matches
  with the version that release-please proposes in the preview-release-notes CI job.

[Bluscream]

Your example workflow will fail if it tries to fix anything inside the .github/workflows/ dir because that requires a github app token. So either show users how to obtain that token or exclude that dir

[ferrarimarco]

Yeah, this is a good point. I'll add a note about this.