add suggested practice for column name for as-of in time-series target data

[nickreich]

https://hubverse.io/en/latest/user-guide/target-data.html#time-series

Looking at the time series file documentation, I wonder if we want to have some recommended practice for a column name indicating an “as of” date in addition to the target or observation date.

One decision that was made for a specific hub is right here:
https://github.com/reichlab/decisions/blob/main/decisions/2025-01-03-ldr-variant-nowcast-target-columns.md