Change to using node-based `markdownlint` `pre-commit` hook

[alexdewar]

This is what we've done elsewhere, e.g.: https://github.com/ImperialCollegeLondon/poetry_template_2/blob/main/.pre-commit-config.yaml

The problem with using a Ruby-based hook is that developers need to have Ruby installed in order to use it, which is particularly irksome for Windows users. Node-based hooks can be installed without any extra steps though (pre-commit will automatically download it).

[alexdewar]

My bad, this has already been implemented on develop. I didn't notice because I was looking at main.