Tutorial notebooks should direct questions to dicourse, not gitter

[GenevieveBuckley]

Notebook 00_overview.ipynb should direct user questions to the Dask discourse forum, instead of the old & unused gitter chat.

Link to the Dask discourse: https://dask.discourse.group/

[GenevieveBuckley]

cc @ian-r-rose (I know you've been doing a lot of work getting more community interaction happening on discourse)