Move Graceful Runner Shutdown Section

What does this MR do?

This MR updates Documentation Files for devopsverify grouprunner. It moves the Graceful Shutdown Section from the Runner Best Practices Page.

Related

gitlab-docs!3178 (comment 1140307970)

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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    • Any task steps should be written as a numbered list.
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• Ensure a release milestone is set.