Resolve "Update the Sales Onboarding with Updated University Content"

I recently updated GitLab University with a new curriculum based layout and added a lot of missing content.

Sid requested that I updated the Sales Onboarding handbook article with the new content.

• I updated all University links to the new source.
• The onboarding article now refers to GitLab University for all theory.
• I tidied up the layout.
• Tweaked some explanations and instructions.
• Removed the code backticks ``` surrounding the tasks so that they are presentable when viewing the handbook.
• Added a link to the raw source to allow a manager to easily copy the tests and tasks for each week.

Please let me know what you think. cc @richardp

Closes #844 (closed)