Mindmap screenshot with pageres-cli

[spier]

Fixes #570. (or at least tries to ... again ;))

I am trying again to fix the GHA workflow that takes a screenshot of the mindmap that is rendered in our online book.
Previous failed tries were #571 and #775.

Found this here that can take screenshots on the CLI:
https://github.com/sindresorhus/pageres-cli

Could not find a GHA that wraps this but it was easy enough to express this in a workflow myself.

TODO:

• once this PR is merged, remember to delete all the old branches of the closed PRs referenced above

[spier]

The screenshot generation works now.

However the "Commit Changes" step does not work, as multiple matrix builds are trying changing the state of the repository in parallel. The git-auto-commit-action GHA explicitly mentions this limitation.

One way to solve this would be to upload the screenshots as artefacts in the matrix build, then at the end have a single job that downloads all artefacts and commits them. See stefanzweifel/git-auto-commit-action#290 and https://docs.github.com/en/actions/how-tos/write-workflows/choose-what-workflows-do/run-job-variations#using-an-output-to-define-two-matrices