Update the timeout/warning info in docs

[emilytheis]

Move content from the attached slide deck into the civiform docs site to help admins set and understand definition of activity and when signout warning modals are triggered

https://docs.google.com/presentation/d/13giTdcehpMiDLDtZVhc1mfhVQc0equfJe43OPeT9jd4/edit?slide=id.p#slide=id.p