Add GitHub Actions check for keywords

It'd be great to have a GitHub Actions check to ensure that tutorial contributions are using the known keyword vocabulary (e.g. as specified in https://github.com/astropy/learn-astropy-librarian/blob/main/astropylibrarian/data/keywords.yaml)

See also [astropy/learn-astropy#](https://github.com/astropy/learn-astropy/issues/42) for a related documentation ticket.