Update "Try in browser" button once tutorial setup is complete

[jsheunis]

Tutorials repo: https://github.com/datalad/tutorials

• [x] create walk-through on same level as study forrest walk-through
• [x] point directly to notebook and not empty binder environment
• [x] fix issue that installs older version of datalad into binder (https://github.com/datalad/datalad-binder/issues/3)

[jsheunis]

Closing this issue, since all items have been addressed a while back already.