Feature: add markdownlint

[Morgy93]

Consider using https://github.com/DavidAnson/markdownlint

[DavidLambauer]

Thank you for the suggestion! We have quite a few static tests in the pipeline. Toxicity checker, spellchecker, and now this one. Fantastic. Would you like to pick up this issue yourself @Morgy93 ? I can see the following task breakdown:

• [ ] Create a GitHub Action that executes the linter on every commit (Maybe just changed files?)
• [ ] Create a first version of the ruleset and discuss it with the docs team (Ideally in the #documentation channel of discord)
• [ ] (optional) Create a small docker run command to run the command locally