Add examples to Data.Traversable

Description

This is a documentation PR that adds some examples to Data.Traversable. Please let me know if there needs to be any improvements/expansions to the examples. I just tried to cover the basic usage of the functions, and demonstrate in broad strokes what the instance methods are doing with an eye towards newcomers.

Tracking: #17929

Since this is a docs MR: @core-libraries @carter

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