feat: Use SveltiaCMS instead of DecapCMS

[MoritzWeber0]

Open issues:

• Footnotes with links are not rendered properly. Have to add another editor component for footnotes or raise an issue with lexical.
• The FIP validity tables do not work because of missing Feature request: Expose field/widget lookup API for custom dynamic field widgets sveltia/sveltia-cms#915. As a workaround, I use relations now, but these do require a page reload after each validity group change, filters do not work.
• Check if page redirect from CMS edit widget can be simplified.
• Update docs

[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

[netlify]

✅ Deploy Preview for fipguide ready!

Name	Link
🔨 Latest commit	d6baec1
🔍 Latest deploy log	https://app.netlify.com/projects/fipguide/deploys/6a855b2619249c00089ed2ec
😎 Deploy Preview	https://deploy-preview-1171--fipguide.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.