[Edit] C++: sets

[codecademy-docs]

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Add new content (definitions, codeblocks, etc.)

Description (optional)

We would like to edit the sets concept entry in c++. The entry lives in content/cpp/concepts/sets/sets.md

Please refer to the term entry template, content standards and markdown style guide when working on the PR for this issue.

Please:

• Add some more examples along with their proper explanations
• Add Best Practices and/or FAQ sections, its uses

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