GitLab Steps: Describe Step Definition

What does this MR do and why?

GitLab Steps: Describe Step Definition

This document describes Step Definition a file that describes how to execute a single referenceable step.

References

• Step Runner Implementation Details (!131688 - merged)
• Blueprint: GitLab Steps - Iterate on technical ... (#423021 - closed)
• Introducing CI Step Syntax (#425879 - closed)

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.