Add example to collections-explained.md showing set operations

[travisjneuman]

Description

The collections concept doc covers sets but could use a practical example showing set operations (union, intersection, difference) with real-world data like comparing two guest lists or finding common items between shopping lists.

Acceptance Criteria

• Add a practical example in the sets section of concepts/collections-explained.md
• Example uses union, intersection, and difference
• Example uses relatable, non-technical data
• No changes to other sections of the doc

[travisjneuman]

Resolved in 36dceb1: Added set operations section (union, intersection, difference, symmetric difference) to collections-explained.md.