Configure a markdown linter for editing our lesson source files

This was mentioned in #437 which I think was prematurely closed. The formatting / linting of markdown files across our lesson contain many inconsistencies, and often leads to me getting sidetracked fixing things that should be enforced by a linter/formatter. `markdownlint` seems like the tool for the job, so I will draft up a config file that enforces The Carpentries Workbench Style and makes editing easier for us.

Start by reviewing issue #437 and the lesson source Markdown files, then compare markdownlint with the Carpentries Workbench Style. Done means a markdownlint configuration exists that enforces the agreed formatting consistently across the lesson sources and supports easier editing.

Written by the indexing model from the issue text.