docs: add GitHub repo homepage

Hey @replicate 👋

This repo is missing a homepage URL in its GitHub metadata.

- [x] Description ✓
- [ ] Homepage URL

This can't be changed via PR, but it's easy to set with the GitHub CLI! 🪄

```bash
brew install gh
gh auth login
gh repo edit replicate/image-editing-arena --homepage "https://image-editing-arena.replicate.dev"
```

Adding a homepage helps visitors quickly find the live site.