Agree/document markdownlint configuration

We currently document adding the markdownlint-cli2 pre-commit hook to projects, however the defaults seem a little on the strict side ((particularly line_length: 80 - there is anecdotal evidence of 99/100 having been used elsewhere). As per uis/devops/webapp-boilerplate#107 (closed), we should discuss/agree a standard .markdownlint-cli2.yaml and document it here.