feat: performance optimizations

[therobrob]

I’ve experimented with some performance optimizations. Feel free to have a critical view. :)

• merged pagefind implementations, because we already overwrite an large part of it. Copilot helped me to reorder the file.
• there was an issue regarding our fonts, which weren’t loaded with font-swap. So i’ve added it and Metric Overrides to avoid layout shifts
• the hero-image wasn’t loaded with fetchpriority=high
• there was an issue with chaining critital requests. I’ve added the defer property to the js.

[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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