Ahodges/documentation/doc764

@WatsonEmily , please distribute for PM review for usability by our audience.
@jhelbert , @stroilova , @shnayder , @rocha  please review for accuracy, completeness, and level of detail, particularly wrt data czars who will need to revise scripts for post-download processing.
@mhoeber , @srpearce please copy edit.
Interesting question @JAAkana. The database exports are snapshots in time, hence every file is the only thing needed to get all the data at that moment in time.

The event exports are a bit different, we used to provide one file with all history of events each weeks, but they became too big to handle by data czars. In this PR we are documenting the new approach, in which we produce one file per day with the events of the previous day. In order for a data czar to get all the data, they will have to download all the files.