Prettier lint md workflow

[wolf99]

Alternative to #129 that uses markdownlint-cli action, this PR instead uses a Prettier action.

Initially implemented with Prettier argument --check to not change files in place, so as to experiment with the configuration.
Intention to later switch from --check to --write.

[wolf99]

OK, I'm out of ideas for how to use actions/checkout properly for this.
Will wait for a response on actions/checkout#1108

[G-Rath]

fwiw, I don't think it's worth the complexity of trying to have CI do commits on PRs anyway 🤷

[wolf99]

Thanks @G-Rath, if no good answer in while from the issue I raised with actions/checkout, then I'll adjust the PR to not make the commit but instead fail if linting is needed.