Add Technical writing course to fullstack onboarding

tl;dr It's a great value add, and we all benefit from it

As discussed in this comment I'm adding the GitLab Technical Writing Fundamentals to the onboarding template.

This course will not only help when writing documentation but also in everyday work when interacting w/ issues & MRs. Participants of the course have found it to a valuable skill to have learned. (Check feedback on the linked issue).