feat: Add news article about SNCB ending on-board ticket sales

[lenderom]

Summary

• Add news article (EN/DE/FR) about SNCB ending all on-board ticket sales from 1 July 2026, including FIP discounted tickets
• Update SNCB operator pages (EN/DE/FR) with an important notice in the "On the Train" / "Im Zug" / "À bord du train" section warning about the upcoming change
• Add new source references [^5] and [^6] to all SNCB operator pages

Closes #789

[github-actions]

Hi there! 👋

Thank you for your contribution to the FIP Guide! 🚀
We appreciate your effort in making rail staff travel information more accessible. 🚄

Checklist before merging:

• Added a description to the Pull Request
• Checked the License of new pictures (non-commercial use without attribution)
• Modified content in English
• Modified content in German
• Modified content in French

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[therobrob]

Why is it an double dash -- here and in lines 16 &28?