Markdownlint rules: test output formatters
We should think about making use of Markdownlint's output formatters: https://github.com/DavidAnson/markdownlint-cli2/blob/main/doc/OutputFormatters.md The code quality formatter would make markdownlint issues show up in the diff (when it works), and the prettier formatter would let us customize the CLI output for readability. It's already in use on the handbook project: - [`.gitlab-ci.yml`](https://gitlab.com/gitlab-com/content-sites/handbook/-/blob/90cd2f892b7cc2968d9b1d35389269b8c18acfe6/.gitlab-ci.yml#L276-282) - [`.markdownlint-cli2.jsonc`](https://gitlab.com/gitlab-com/content-sites/handbook/-/blob/90cd2f892b7cc2968d9b1d35389269b8c18acfe6/.markdownlint-cli2.jsonc#L2-15)
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