Added ChemIllusion to the list of AI tools.

📝 New Tool Information

• ChemIllusion
• AI figure tool that knows Chemistry and Biology
• https://chemillusion.com

📌 Description

✅ Checklist

• [✅ ] I have read the Contribution Guidelines
• [✅ ] Only one tool is modified in this PR
• [ ✅] All required fields (name, description, URL, altern.ai link if available) are provided
• [✅ ] The tool link is valid and accessible
• [✅ ] The tool is not already listed
• [✅ ] The tool is placed in the correct category
• [✅ ] The tool is added at the end of its category
• [✅ ] No unrelated changes included in this PR

🛠 Type of Change

• 📚 Documentation / README update
• [✅ ] ➕ Adding a new AI tool
• 🗑 Removing a deprecated/invalid AI tool
• ♻️ Refactoring or reorganizing existing entries
• 🐛 Bug fix
• Other (please specify):

📷 Screenshots / Demo (if applicable)

💡 Additional Notes