[Website]  Possibly missing item reference - auxr from CENC

### Summary

ISO/IEC 23001-7:2016 Amd.1:2019 Section 8.4.1 describes linking items using an `auxr` item reference. However that does not appear to be in the list of registered item references

### Steps to reproduce

1. Check spec identified above.
2. Check list at https://github.com/mp4ra/mp4ra.github.io/blob/main/data/item-references.csv