Consider adding a linter into the build process

[at055612]

Add either https://github.com/markdownlint/markdownlint (ruby) or https://github.com/DavidAnson/markdownlint (node) into the CI build. The node one is copied from the ruby one but not sure which is the best or most active. Both would need configuration to according to our preferred style of .md and also to not barf on Hugo stuff like short codes or front matter.