Add OpenAPI documentation

[balintking]

Description

Integrate Swagger/OpenAPI documentation into the project to provide clear and interactive API documentation for developers.

Read docs: https://docs.nestjs.com/openapi/introduction

Tasks

• Install Swagger dependencies in the project.
• Configure Swagger for the Next.js project.
• Ensure the documentation is accessible via a specific route.
• Document the OpenAPI access route in the README file.

Acceptance Criteria

• Swagger/OpenAPI documentation should be available and accessible.
• All existing API endpoints should be documented clearly.