i18n(pl): update islands.mdx

[erbierc]

Description (required)

I updated islands.mdx to include information about server islands. I also removed some old content and translated things that were added to the English version after this file was initially translated.

Related issues & labels (optional)

• Closes #N/A
• Suggested label: i18n

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pl/concepts/islands.mdx	Localization changed, will be marked as complete.

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.