Document how to run WhisperBack from the Activities overview

As discussed in #11061 (closed) this could help people to give WhisperBack reports.
(We decided to add this ticket in the 201608 monthly meeting and to set priority to low)

Related issues

Related to #13299 (closed)

Original created by @muri on 11621 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information