CI: Use Netlify for notebooks tests

[ViktorvdValk]

Update the CI for notebook tests to use Netlify and Nbmake. A NETLIFY_TOKEN and a NETLIFY_SITE_API_ID still have to be added to the secrets of the ITKElastix repo. I suppose this it's better to make an account from within ITK for this and use that or shall I add my token and api_id to the ITKElastix secrets?