Add a pre-commit hook to format the Markdown files

[jhlegarreta]

Adding a pre-commit hook that is able to ensure that the Markdown files have a consistent format and follow the carpentries style would be a great addition and would allow developers to concentrate on the contents rather than the formatting.

Best would be to use a carpentries-wide adopted/recommended tool and the same configuration file.

[jhlegarreta]

Cross-referencing #85 (comment) and #143.

Not sure if markdownlint is able to actually apply the format to the files or else it just flags the non-conforming lines; prettier seems to do both jobs.