[SE-2763] Link to the handbook when mentioning how JIRA works

Send newcomers to a recently expanded section of the handbook (public!177 (merged)) that includes how to create tasks, which fields to use and what do the colors and icons in JIRA mean. Reading all details isn't required and isn't needed by all newcomers, therefore they're in the handbook as reference.

Testing:

• check onboarding-dev (I already placed it there), https://studio.courses.opencraft.com/container/block-v1:OpenCraft+onboarding-dev+course+type@vertical+block@first_sprint_task_tracker
• see the change in context: check that there's that new paragraph

After merging, deploy the new course.