Content review of FITS file tutorials

Review the content and provide updates that are relevant for an undergraduate audience.

* [Viewing and manipulating data from FITS tables](https://learn.astropy.org/tutorials/FITS-tables.html)
* [Viewing and manipulating FITS images](https://learn.astropy.org/tutorials/FITS-images.html)
* [Edit a FITS header](https://learn.astropy.org/tutorials/FITS-header.html)
