feat: document new sitemap chunks feature

[Princesseuh]

Description (required)

@astrojs/sitemap included a new chunks feature for sitemaps, but it got merged without docs. Sorry!

Related issues & labels (optional)

withastro/astro#14471

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en/guides/integrations-guide/sitemap.mdx	Source changed, localizations will be marked as outdated.

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[sarah11918]

Thanks @Princesseuh! Just a quick clarifying comment below (and one for Armand, which can be handled separately, but wanted him to see this)

[sarah11918]

(No merge-on-release label because feature is already released!)

[ArmandPhilippot]

Well written, Erika! I just left a nit regarding the syntax highlight (since we are not highlighting sitemap({, we should not highlight the closing braces).

And, good thing I'm curious! GitHub did it again... no notif for Sarah's ping... 😅 Good idea, I add that to my TODO list!

[sarah11918]

Looks fantastic @Princesseuh ! Thank's for catching this missing docs content!