add markdown linting features

What does this MR do?

It adds markdownlint (https://github.com/DavidAnson/markdownlint) for basic markdown linting features

How confident are you it won't break things if deployed?

It's a dev dependency so I'd go for 99,98%

How to test

Dedicated documentation is filed in another MR: !2017 (merged)

For this simply run: scripts/lint-markdown

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

Added basic markdown linting features