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

[wiktrek]

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• [x] I have followed every step in the contributing guide (updated 2022-10-06).
• [x] The PR title follows the convention we established conventional-commit

---

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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Name	Status	Preview	Comments	Updated (UTC)
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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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[juliusmarminge]

This is outdated (sorry for not reviewing) so converting to draft. Ping me on Discord when it's ready