docs: expand sample() example to show sampling from a filtered subset

[Wnayar]

Summary

The Aggregate#sample() docs currently show only a single example sampling from the whole collection. This adds a second example demonstrating .match().sample(), which samples from a filtered subset rather than the whole collection. This is a common use case, and the stage ordering (filter before sample) is worth showing explicitly.

Examples

// picks 3 random documents from the whole collection
aggregate.sample(3);

// `$match` before `$sample` samples from the filtered subset
aggregate.match({ difficulty: 'easy' }).sample(3);