Add markdownlint-cli2 to project

When I was making a content contribution to the project, I noticed that we were linting Markdown, but in a very limited fashion.

I wondered if we might replace that with a more comprehensive linter that includes a rule for that which was already being linted for: https://github.com/DavidAnson/markdownlint/blob/main/doc/md047.md and more.

Closes #506 (closed)