Add mdx linting to match docs repo

The docs repo has been updated to include stricter linting on markdown files. This means docs won't be updated if the docs parser starts getting picky.

@taylordowns2000 likes the stictness of the docs parser because it catches genuine errors. So the only solution I can see to stop docs builds failing silently is to introduce matching linting here in this repo.

So we need:

• To copy/duplicate the linting settings from the docs repo into here
• Add a new script like lint:docs to this repo
• Run the new script in CI. The build must fail if docs errors are encountered