docs: added polish translation to folder-structure-app.mdx

[wiktrek]

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---

Changelog

• Added polish translation to folder-structure-app.mdx
• changed link in en/folder-structure-app.mdx to next.js app router docs
• added IntroductionTab component in pl/introduction.mdx
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Hey t3-oss/translators!

This PR contains changes to your language. Please review the changes ❤️.

PL: @matibox, @Infiplaya, @PiotrekPKP

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