docs: Update Discord link in README.md

[sairin1202]

📝 Pull Request Summary

Please provide a short summary explaining the purpose of this PR.

---

✅ What does this PR do?

• Clearly describe the change introduced.
• Mention the motivation or problem it solves.

---

🤔 Why is this change needed?

• Explain the context or user impact.
• Link any relevant issue or discussion.

---

🔍 Type of Change

Please check what applies:

• Bug fix
• New feature
• Documentation update
• Refactor / cleanup
• Other (please explain)

---

✅ PR Quality Checklist

• PR title follows the conventional format (feat:, fix:, docs:)
• Changes are limited in scope and easy to review
• Documentation updated where applicable
• No breaking changes (or clearly documented)
• Related issues or discussions linked

---

📌 Optional

• Screenshots or examples added (if applicable)
• Edge cases considered
• Follow-up tasks mentioned

[Copilot]

Pull request overview

Updates the README to point users to the current Discord invite URL, keeping community links accurate.

Changes:

• Replaced the legacy discord.gg/memu invite URL with discord.com/invite/hQZntfGsbJ in the Discord badge.
• Updated the “Discord Community” quick link to the new invite URL.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.