feat: Add pdf exporter configuration

[ewuerger]

This PR implements a cover page and a CSS stylesheet for the Polarion PDF Exporter plugin.

[github-actions]

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[micha91]

Why do we include the weasyprint dockerfile here? Beside that LGTM

[ewuerger]

Why do we include the weasyprint dockerfile here? Beside that LGTM

This was already approved, but I can remember that we argued to not version the extension config in here. Is that still true?