Write cloud transition documentation

## Description
Create comprehensive documentation guiding developers through transitioning from local AI to cloud providers, covering configuration patterns, security best practices, and migration strategies.

## Goals
- Explain seamless local → cloud transition
- Document security best practices
- Provide configuration patterns
- Address common questions and concerns

## Documentation Sections

### 1. Cloud Transition Overview
- Why transition to cloud
- When to transition
- What changes are needed (config only, no code)

### 2. Configuration Patterns
```csharp
// Debug vs Release
#if DEBUG
builder.Services.AddAIChatClient();
#else
builder.Services.AddSingleton(new AzureOpenAIChatClient(...));
#endif

// Environment-based
// appsettings.json patterns
```

### 3. Security Best Practices

- API key management (DO NOT commit)
- Using Azure Key Vault
- Using App Configuration service
- SecureStorage for mobile apps
- Environment variables
- CI/CD secrets management

### 4. Provider Setup Guides

- Azure OpenAI setup
- Other MEAI providers
- Authentication patterns
- Endpoint configuration

### 5. Migration Checklist

- [ ] Set up cloud provider account
- [ ] Create API keys
- [ ] Store keys securely
- [ ] Update configuration
- [ ] Test functionality
- [ ] Deploy

### 6. Troubleshooting

- Common configuration errors
- Authentication failures
- Network issues
- Cost monitoring

## Notes

- Emphasize zero code changes required
- Security is critical - highlight best practices
- Include links to Azure documentation
- Address cost considerations