Move reference documentation to the handbook - Onboarding Course

The onboarding course contains the information newcomers need to learn to get started, while the handbook is meant as a reference throughout the duration of the job. So the relevant documents which were contained in the course have been moved to the handbook: public!5 (merged)

The course with the contents of the current branch is visible at: https://courses.opencraft.com/courses/course-v1:OpenCraft+onboarding-dev+course/courseware/chapter_newcomer/introduction_newcomer/

See detailed description, other related PRs and the recommended order to review these on the main forum post: https://forum.opencraft.com/t/opencraft-v3-scaling-up-some-self-management/141