Update SDK documentation for new type system

[nickna]

Overview

Part of #658 - Document the new type system and provide migration guidance for developers.

Dependencies

Should be done after most refactoring is complete to ensure accurate documentation.

Documentation Needed

1. Architecture Documentation

• Explain why we use generated types as source of truth
• Document the type generation process
• Show the relationship between OpenAPI spec and TypeScript types

2. Developer Guide

• How to work with generated types
• When to extend vs alias types
• Common patterns and anti-patterns
• How to handle client-only types

3. Migration Guide

• Breaking changes list
• Step-by-step migration for consumers
• Common migration issues and solutions
• Type compatibility reference

4. Maintenance Guide

• How to regenerate types
• Adding new type validations
• Troubleshooting type issues
• CI/CD type checking process

Documentation Structure

docs/
├── sdk/
│   ├── architecture/
│   │   └── type-system.md
│   ├── guides/
│   │   ├── working-with-types.md
│   │   ├── migration-guide.md
│   │   └── type-patterns.md
│   └── reference/
│       ├── generated-types.md
│       └── type-mappings.md

Example Content

Type System Overview

# SDK Type System

The Admin SDK uses generated TypeScript types from the OpenAPI specification as the single source of truth.

## Why Generated Types?

1. **Consistency**: API and SDK types always match
2. **Automation**: No manual updates needed
3. **Type Safety**: Catches API changes at compile time

## Type Patterns

### Simple Alias
\`\`\`typescript
export type ProviderCredentialDto = components['schemas']['ConduitLLM.Configuration.DTOs.ProviderCredentialDto'];
\`\`\`

### Extended Types
\`\`\`typescript
interface ProviderWithHealth extends ProviderCredentialDto {
  healthStatus?: 'healthy' | 'unhealthy';
}
\`\`\`

Definition of Done

• All documentation sections complete
• Code examples tested and working
• Migration guide validated by team
• Documentation reviewed for clarity
• Published to documentation site
• Team trained on new patterns