docs: add explanation about layout usage in error.vue

[GalacticHypernova]

🔗 Linked issue

#25320

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Following #25320 (comment) and the discussion in the Discord, this adds a few words about using layouts in error.vue.
Originally meant to be bundled in the same PR, I unfortunately wasn't there in time for that merge

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.

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