Add Markdown linting to project

What does this MR do and why?

We currently don't have linting coverage on Markdown files in this project.

We currently mostly use markdownlint-cli for Technical Writing rather than markdownlint-cli2, but I think we should move across completely because the person responsible for the markdownlint library is also responsible for markdownlint-cli2.

I think we'll eventually switch to markdownlint-cli2 in documentation projects, but this is a good place to start.

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Run yarn install to make sure you have the right dependencies.
4. Run make lint-markdown. It should run without errors.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.