[documentation] Information about region numbering

[MatthewPierson90]

It would be useful if it were mentioned in the documentation that regions have to be 6 digits with 0's as place holders.

[jgrss]

Good point. I show that below, but didn't make it clear that it is mandatory. I'll fix that.

The years in the directories must align with the training data files. More specifically, the training data year (year in the polygon/grid pairs) should correspond to the time series end year. For example, a file named 000001_poly_2020.gpkg should be trained on 2019-2020 imagery, while 000001_poly_2022.gpkg would match a 2021-2022 time series.