feat: add spa-loading-template.html to content config

[nikolasdas]

🔗 Linked issue

resolves #906

❓ Type of change

• [x] 📖 Documentation (updates to the documentation or readme)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality)
• [ ] ✨ 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

This simply adds spa-loading-template.html to the default content config and updates the docs accordingly