add examples of how to use or adopt desispec.zcatalog.find_primary_spectra

In the DR1 paper, we write:

```
For analyses seeking the “best” redshift for a given target, regardless of the DESI-specific survey/program combination,
we recommend the zall-pix-iron.fits file, which combines all the HEALPix-based redshifts across all programs into a single
file. The “best” redshift can be retrieved using the ZCAT PRIMARY boolean column, which is derived using the Python
script desispec.zcatalog.find_primary_spectra. This same script can be adopted to subselect multiply-observed targets
from a custom selection of spectra to determine the recommended redshift.
```

It would be useful to make sure at least one tutorial demonstrates the proposed action.