Install markdownlint-cli2 globally before running whitespace tests

What does this MR do and why?

The test_EOL_whitespace job fails because markdownlint-cli2 can't be found: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3799612147.

This ensures MR globally available for testing all the different projects.

CC @jglassman1 @msedlakjakubowski @axil @sarahgerman

How to set up and validate locally

1. Set up Docker environment.
2. Run: docker run -ti --rm registry.gitlab.com/gitlab-org/gitlab-docs/lint-html:alpine-3.16-ruby-3.0.5-869cfc5d bash
3. In the Docker container, run the following:
   1. apk add jq.
   2. bundle config set --local deployment true.
   3. bundle install.
   4. bundle exec rake default.
   5. yarn global add markdownlint-cli2
   6. make markdownlint-whitespace-tests
4. See tests run successfully.

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