Add link to SEP tutorial on main SEP page.

What does this MR do and why?

This change adds a new tutorial link to the documentation page about scan execution policies. The update includes a reference to a tutorial that teaches users how to create scan execution policies, providing an additional learning resource alongside the existing video walkthroughs. This gives users another way to learn about setting up these security scanning policies in GitLab that doesn't require the use of videos.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/578017

Screenshots or screen recordings

Before	After

How to set up and validate locally

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