Create stable link to flat version of data model

[aclayton555]

Ideally, create a stable link to the flat version of the data model csv: https://flatgithub.com/ncihtan/data-models/blob/v23.8.1/HTAN.model.csv?filename=HTAN.model.csv&sha=e7abfe4ef5737f48362a3b1cc542f49436be9cf3

This stable link should reflect the latest, released version of the data model.

Where appropriate, update data ingress docs (e.g. FAQs) with this link to serve as a more user-friendly, contributor view of the data model (so folks don't have to go digging through release assets or in github)