Fully implement markdownlint-cli2 in project

What does this merge request do and why?

Some time ago, we moved this project to only use markdownlint-cli2 rather than markdownlint-cli: !2780 (merged).

However, we never adopted the new-style of configuration: https://github.com/DavidAnson/markdownlint-cli2#configuration. It's very similar, but with a slightly different structure and requirements for configuration filename.

This MR makes that move to the new style of configuration with a view to eventually moving all projects to that configuration style, and removing the original markdownlint-cli from our projects.

While here, I realized we didn't have a yarn script for linting Markdown, which meant falling back to a globally-installed version instead of using the version provided with the project. So I've added that. Using a project version means we don't need a custom Docker image for the test.

Related to gitlab-docs#1761 (closed)

CC @axil @aqualls @marcel.amirault @sarahgerman just for awareness.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.