i18n(pl): Add Polish navigation

[erbierc]

Description (required)

I added the Polish translation for docs navigation. When in doubt, I referred to the Russian translation (my limited knowledge of that language was helpful in that regard). I also added comments with original English strings to make reviewing easier.

Related issues & labels (optional)

• Closes #N/A
• Suggested label: i18n

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src/content/nav/pl.ts	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.

[yanthomasdev]

LGTM! Thanks!