Consider adding a linter into the build process

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.

Compare the Ruby and Node markdownlint options linked in the issue, starting with the repository's existing CI build configuration. Define the preferred Markdown rules and Hugo exceptions for shortcodes and front matter, then verify that the CI build lints the documentation without rejecting valid Hugo content.

Written by the indexing model from the issue text.