docs(contributing): improve contributing guide clarity and completeness

[francescobianco]

This pull request updates the contribution guidelines in the docs/contributing.md file to better reflect the current project setup and improve clarity for new contributors. The instructions have been revised to match the OpenAPI PHP SDK, and additional steps and details have been introduced.

Updates to project instructions:

• Changed the project name references from "Openapi SDK" to "OpenAPI PHP SDK" and updated the repository URL in the clone command.
• Added instructions for installing dependencies with composer install and running tests with PHPUnit before committing changes.

Improvements to guidelines and clarity:

• Clarified coding style requirements, specifying PHP 8.0+ and English for code, comments, and documentation.
• Improved instructions for reporting issues, including clearer descriptions and relevant error output.
• Updated the thank you message to match the new project name.## 📋 Description

Brief description of the changes made in this PR.

✨ Type of Change

• 🐛 Bug fix (fixes an issue)
• ✨ New feature (adds functionality)
• 🔧 Refactoring (code restructuring without functional changes)
• 📚 Documentation (updates to documentation)
• 🧪 Tests (adding or modifying tests)
• 🔒 Security (security-related fixes)
• ⚡ Performance (performance improvements)
• 🎨 Style (formatting changes, no logic changes)

🔍 Main Changes

🧪 Testing

• I have tested the changes locally
• I have added/updated unit tests
• All tests pass (./vendor/bin/phpunit)
• I have verified PHP 8.0+ compatibility

📝 Additional Notes

Any additional information, considerations, or important context for reviewers.

🔗 Related Issue

Closes #[issue_number] (if applicable)

📸 Screenshots (if applicable)

If changes affect the interface or visible behavior, include screenshots.

✅ Checklist

• My code follows the project conventions
• I have performed a self-review of my code
• I have commented complex code where necessary
• Documentation has been updated (if needed)
• I have not introduced breaking changes (or documented them)
• I have verified no sensitive information is in the code