CI: fix markdownlint script
Since the while loop ran in a subshell, changes to the MDL_FAILED variable aren't visible to the parent, so the job always succeeded. Woops
Fix it and reflow markdown to pass. I highly recommend using an IDE plugin for markdownlint while working on markdown files as it can show inline warnings (in vscode if you turn them on!). A markdown formatter is also good ("format document" in vscode).