DOCS: Add API Reference Placeholder for Game & User Services

[AndersonGACFilho]

Summary

Create placeholder markdown files in the docs/api/ directory for the Game Service and User Service, to be populated later with detailed OpenAPI/Swagger documentation.

Motivation

To establish the structure for future API documentation.

Scope

In-scope:

• Creating docs/api/game-service.md.
• Creating docs/api/user-service.md.
• Adding a brief note in each file that it is a placeholder.

Out-of-scope:

• Generating or writing the actual API documentation.

Acceptance Criteria

• The placeholder files are created in the correct location.
• Content accurate & reviewed.

Affected Files

• docs/api/game-service.md
• docs/api/user-service.md

Checklist

• Labels added (docs, component/game-service, component/user-service)
• CHANGELOG updated if user-visible
• No sensitive info added