Add repository-based instructions for GitHub Copilot

[firasrg]

Add repository-based instructions for GitHub Copilot, to guide code suggestions for TJ Bot. This will align Copilot with our coding standards and project needs, improving quality and contributor experience.

Motivation

For TJ Bot, a Discord command-based bot built with Java and JDA, custom instructions can :

• Ensure consistent code style and patterns.
• Speed up development with relevant Copilot suggestions.
• Help contributors learn project-specific best practices.
• Reduce errors by highlighting common pitfalls.

ToDo

• Create .github/copilot-instructions.md with information like :
  • Project overview (Gradle, Java, JDA, commands, etc..).
  • Coding standards (e.g., camelCase, Google Java Style Guide, Javadoc).
  • Patterns for features or bugfixes.
  • Language version and libraries.
• Get community feedback (Discord/GitHub).
• Test Copilot suggestions.
• Align with other guidelines like in CONTRIBUTING.md.