[FEATURE] md linting #228

Writing design documents in markdown is here to stay: I noticed that my PR reviews run some linter (example). It would be usefull to also include it in the dev container and linting step.

See if this is a good idea:: https://github.com/DavidAnson/markdownlint-cli2 (and probably requires a lot of doc changes so that's why it should be it's own PR at some point).