DOC Change slack invitation from email to link
Simplified the slack invitation process after discussion with @rhshadrach and @MarcoGorelli.

The current email process is maintenance heavy and until now, we received very little spam. We want to try letting people join themselves and see if spam increases or not.

Local build looked correct.
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/52014/ I think we can merge now, tag has been pushed