docs: document fallback pages provided by the service

[MikkelHebel]

Summary

Adds a ## Pages section to the README documenting every page this service can serve, what triggers each one, and what it looks like.

• Lists all generated pages with their title and intended use
• Documents the two client-side states behind every page (valid challenge-instance subdomain "loading" state with auto-reload polling; anything else gets a generic "Invalid URL!" message)
• Adds screenshots for each page/state under docs/images/, captured by running generator.py

This closes #16

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[MikkelHebel]

I have read the CLA Document and I hereby sign the CLA