Adding a linter with a basic configuration

[gkunz]

This adds a markdown linter as a GitHub action to ensure that the texts which are being added use consistent and correct markdown syntax.

[myteron]

There are two formatting issues on the main /readme.md that would block deliveries.

1. forced width of tables, looks horrible without
   would need a solution or exclude MD033 inline haml and MD045 missing alt string on image
   Note that in 'theory' MD supports dots for spaces but is practically not working. Neither did I find anything else working.

2. Forced newlines in cells.
   would need a solution or exclude MD033 inline html
   In theory MD supports .. or \ for forced nl but in practice table rows required a continues line, so can't newline. Besides \ +nl or .. + nl does not work anyways as it seems.