Add eagerness option description and example for prefetch()

[Marocco2]

Description (required)

As per request, I've added a new section for Prefetch docs page describing what the new option does and a use case that it would be useful

For Astro version: 5.6. See astro PR #13405

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en/guides/prefetch.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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