fix: clarify Stena Line BV booking requirements

[lenderom]

What

• add date of birth as required booking detail for Stena Line BV email/phone in de/en/fr
• clarify conditional required details in the main list:
  • only for FIP 50 tickets: copy of FIP Card
  • only for reservations with FIP Coupon: coupon number
• keep reservation note as warning highlight, including that booking is only possible once the FIP Coupon/Freifahrtschein is available

Files

• content/booking/stena-line-bv-email/index.de.md
• content/booking/stena-line-bv-email/index.en.md
• content/booking/stena-line-bv-email/index.fr.md
• content/booking/stena-line-bv-phone/index.de.md
• content/booking/stena-line-bv-phone/index.en.md
• content/booking/stena-line-bv-phone/index.fr.md

Fixes #995

[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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