[pug/de-de] Fix file extension in instructions about include

[AdaiasMagdiel]

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Adjusting file paths in the German language file to match the updated Pug file names for consistency and clarity of the project structure.

• Updated references from "index.png" to "index.pug"
• Updated references from "includes/nav.png" to "includes/nav.pug"

[vendethiel]

Thanks!