use GitHub action workflow to check Markdown

[sverhoeven]

Pull Request checklist

We appreciate your time and effort to improve the tutorial. Please keep in mind that lesson maintainers are volunteers and it may be some time before they can respond to your contribution.

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Before you start

• Read CONTRIBUTING.md.
• Create an issue to discuss your idea. This allows your contributions to be incorporated into the tutorial.

Tasks

• Give this pull request a descriptive title.
• If you are contributing to existing lesson materials, please make sure the content conforms to the Lesson development section in CONTRIBUTING.md and does not contain any spelling or grammatical errors.
• If you are making a new episode, please make sure the content conforms to the Lesson organization and Lesson formatting sections in CONTRIBUTING.md and does not contain any spelling or grammatical errors.
• Preferably Codacy checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why.
• Preview changes on your machine before pushing them to GitHub by running make serve, alternatively make docker-serve. Please see the Previewing your changes locally section in CONTRIBUTING.md for installation instructions.
• All code instructions have been tested.

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

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Closes {Link to the corresponding issue}

[Peter9192]

Why build for push and PR? Wouldn't one of them be enough?

[sverhoeven]

GitHub action fails as expected due to check errors, see log of GitHub action job to see the errors.

[sverhoeven]

Why build for push and PR? Wouldn't one of them be enough?

Push is for commits to this repo and PR also runs for external PRs. So we need both.

[sverhoeven]

Once merged we should fix the lint errors and disable codacy.

[Peter9192]

Very nice @sverhoeven! I'll merge this, open a new issue for existing problems, and disable codacy.

Cheers!