Add interface guidelines for river breakout content length

[danielguillan]

Adds content length guidance and do's and don'ts for river breakout.

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Latest commit: add8643

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[github-actions]

🟢 No design token changes found

[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences.

Please review the differences by using the test artifacts to ensure that the changes were intentional.

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Review visual differences

[auareyou]

Looks good!

[nsolerieu]

Great do/dont visual examples