improve pre-commit checks when example notebooks are removed

[drbenvincent]

When removing an example .ipynb file, the pre-commit checks do not check that the corresponding myst.md file has been deleted. Could add a check that there are no myst.md files without corresponding .ipynb files.

[OriolAbril]

pre-commit check also seems to pass if there are myst without associated ipynb examples when again, it should fail