Draft: Add sm margin bottom to spacing mixin

What does this MR do?

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
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  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.