[STAR-3054] Add latest sales training materials to onboarding course
This MR adds a new unit in the onboarding course, describing and linking to materials from Douglas' "Effective Client Communication" training.
Testing instructions:
- download course directory as a tar archive
- import archive in Dev onboarding course in Studio
- locate new unit (found at Course -> Confirmed core team member -> Sales -> Effective Client Communication)
- review copy and links
Edited by Gabriel D'Amours