feat: Added extra option to add dynamic sitemap-indexes without them being added to the main sitemap-index

[peti446]

What's the change

I added an extra option robotsTxtOptions.additionalSitemapsIndex that allows users to set indexes they want to add to the robots.txt but not add to the general sitemap.xml as this is the current implementation causing errors as sitemap index cannot have other sitemap indexes

Closes

https://github.com/iamvishnusankar/next-sitemap/issues/737

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[peti446]

@iamvishnusankar in case you want to merge this one ^^

[github-actions[bot]]

Closing this PR due to inactivity.