Adds Markdown Linter

[vpetersson]

• Adds Markdown linter
  • I used line lenght of 80, which appears to be what @oej has been using
• Reformatted all Markdown files (which found a few errors too)

[oej]

Brilliant! I like having a linter and since we have markdown now, I think it's a good idea. I'll take a more detailed look later this weekend.

[madpah]

@vpetersson - any reason you have chosen not to use an existing GitHub Action for this?

• https://github.com/DavidAnson/markdownlint-cli2-action
• https://github.com/super-linter/super-linter