Update singleton pattern docs in design_patterns.md
What does this MR do?
Updates single pattern docs in design_patterns.md as suggest in a recent frontend call.
This change promotes "keep it simple".
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Why was this MR needed?
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Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated -
API support added - Tests
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Added for this feature/bug -
All builds are passing
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Conform by the merge request performance guides -
Conform by the style guides -
Branch has no merge conflicts with master(if it does - rebase it please) -
Squashed related commits together