Update the Guide to indicate study data export is currently disabled by default

[mbaumann-broad]

Discussed in Slack here.

The Guide should be updated to indicate study data export is currently temporarily disabled by default.

Here's some strawman draft text:

Note: The format of exported study data is currently being updated.
Exporting study data is temporarily disabled by default while this work is in progress.
Export will be reenabled when the format is stable and working well.