docs(en/sitemap.mdx): describe `customSitemaps`

[gouravkhunger]

Description (required)

Add docs for the new customSitemaps option in the @astrojs/sitemap integration.

Related issues & labels (optional)

• withastro/astro#13682
• Suggested label: add new content

First-time contributor to Astro Docs?

Yes, discord: @gouravkhunger

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Tracked Files

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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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