Adds scrum tutorial to docs.gitlab.com

What does this MR do and why?

This MR adds a tutorial on how to use GitLab to facilitate typical Scrum ceremonies and activities.

Screenshots or screen recordings

There are no changes to the application. This is a documentation-only change.

How to set up and validate locally

Visit the docs site locally and go to /ee/tutorials/, then select the Scrum tutorial.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.