Clarify when Effects are needed for data fetching (#8226)

[kanikaa88]

This PR improves the explanation in the "Fetching data" section of "You Might Not Need an Effect" to clarify when an Effect is needed versus when it's not.

The previous text was confusing because it said "You don't need to move this fetch to an event handler" which seemed contradictory given the page's title. This change:

• Clearly states that this IS a case where an Effect IS needed (for synchronization with external system)
• Explains the key difference: Effects are needed when synchronizing with external systems, regardless of how the state changes
• Adds a contrast example showing when you would use an event handler instead

This makes it clearer for readers to understand when Effects are appropriate for data fetching.

Closes #8226

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