Add comprehensive provider implementation guide

Summary

This PR adds a detailed guide for implementing new cloud providers in the Brev Cloud SDK. The guide is based on the well-tested Lambda Labs implementation and provides step-by-step instructions for creating new provider integrations.

Key additions:

• docs/HOW_TO_ADD_A_PROVIDER.md: Comprehensive 400+ line implementation guide covering all aspects of provider development
• Updated README.md: Added link to the new provider guide in the Documentation section

The guide covers:

• Complete CloudClient interface implementation patterns
• NotImplCloudClient pattern for handling unsupported features
• Capabilities declaration and security implementation
• Instance lifecycle management and type conversion
• Documentation requirements (README, SECURITY.md, CONTRIBUTE.md)
• Testing strategies and common pitfalls
• Code examples and reference to Lambda Labs implementation

Review & Testing Checklist for Human

• Verify technical accuracy - Compare guide examples with actual Lambda Labs implementation in internal/lambdalabs/v1/ to ensure patterns and interfaces are correctly documented
• Check interface completeness - Ensure all CloudClient sub-interfaces mentioned in the guide actually exist in pkg/v1/ and are accurately described
• Test documentation links - Verify all internal links to files, interfaces, and documentation work correctly
• Validate security guidance - Ensure security implementation instructions align with docs/SECURITY.md requirements
• Review code examples - Check that provided code snippets compile and follow project conventions

Recommended test plan: Walk through the guide as if implementing a new provider to identify any missing steps or unclear instructions.

Diagram

%%{ init : { "theme" : "default" }}%%
graph TB
    README["README.md<br/>(Main project docs)"]:::minor-edit
    HOW_TO_ADD["docs/HOW_TO_ADD_A_PROVIDER.md<br/>(New comprehensive guide)"]:::major-edit
    
    LAMBDA_CLIENT["internal/lambdalabs/v1/client.go<br/>(Reference implementation)"]:::context
    LAMBDA_INSTANCE["internal/lambdalabs/v1/instance.go<br/>(Instance operations)"]:::context
    LAMBDA_CAPS["internal/lambdalabs/v1/capabilities.go<br/>(Feature declarations)"]:::context
    
    PKG_CLIENT["pkg/v1/client.go<br/>(CloudClient interface)"]:::context
    PKG_NOTIMPL["pkg/v1/notimplemented.go<br/>(NotImplCloudClient pattern)"]:::context
    
    SECURITY["docs/SECURITY.md<br/>(Security requirements)"]:::context
    
    README --> HOW_TO_ADD
    HOW_TO_ADD --> LAMBDA_CLIENT
    HOW_TO_ADD --> LAMBDA_INSTANCE
    HOW_TO_ADD --> LAMBDA_CAPS
    HOW_TO_ADD --> PKG_CLIENT
    HOW_TO_ADD --> PKG_NOTIMPL
    HOW_TO_ADD --> SECURITY
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#F5F5F5

Notes

This documentation-only change carries low risk of breaking existing functionality but requires careful review for technical accuracy. The guide serves as the primary resource for future provider implementations, so ensuring correctness is critical for developer success.

Link to Devin run: https://app.devin.ai/sessions/b2031a662469495385f575d0d420561e
Requested by: Alec Fong (@theFong)

All lint checks and tests pass successfully. The guide references the Lambda Labs implementation as the canonical example, which was specifically chosen by the user as the "best working example currently" with "well tested" GitHub Actions integration.