feat: add comprehensive development guidelines document

[gustaavik]

This pull request introduces a comprehensive development guidelines document for the Commercify e-commerce backend API. The document outlines best practices, architectural principles, coding standards, and other essential guidelines to ensure code consistency, maintainability, and adherence to project architecture.

Added Development Guidelines

• Architectural Principles: Defines the Clean Architecture layers (Domain, Application, Infrastructure, Interfaces) and the dependencies rule to ensure proper separation of concerns. (.github/copilot-instructions.md, .github/copilot-instructions.mdR1-R269)
• Code Structure: Provides a directory structure for organizing the project, including layers for API, application logic, domain entities, and infrastructure. (.github/copilot-instructions.md, .github/copilot-instructions.mdR1-R269)

Coding Standards and Practices

• General Coding Standards: Specifies formatting, naming conventions, error handling, and function documentation practices. (.github/copilot-instructions.md, .github/copilot-instructions.mdR1-R269)
• Database Practices: Includes guidelines for migrations, query safety, indexing, and transaction management. (.github/copilot-instructions.md, .github/copilot-instructions.mdR1-R269)

API and Testing Guidelines

• API Design: Covers RESTful principles, versioning, security, and a standardized format for documenting API endpoints. (.github/copilot-instructions.md, .github/copilot-instructions.mdR1-R269)
• Testing: Emphasizes unit and integration testing with mocks, test databases, and cleanup practices. (`.github/copilot

[Copilot]

Pull Request Overview

This pull request introduces a comprehensive development guidelines document for the Commercify e-commerce backend API. The document details architectural principles, code structure recommendations, coding standards, and testing practices to ensure consistency and maintainability.

• Introduces Clean Architecture principles and dependency rules.
• Outlines a recommended directory structure.
• Specifies guidelines for coding standards, error handling, testing, and API documentation.