docs: add isMalicious threat intelligence platform

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Summary

Add isMalicious to the Threat Intelligence section.

About isMalicious

isMalicious is a threat intelligence platform that:

• Aggregates malicious IP and domain data from multiple security feeds
• Provides real-time reputation scoring (0-100 confidence score)
• Categorizes threats (phishing, malware, C2, botnet, ransomware, spam, scam)
• Offers integrations with OpenCTI, MISP, and browser extensions
• Free API tier available

Checklist

• Read the contribution guidelines
• Searched for duplicates
• Added to the bottom of the relevant category (Threat Intelligence)
• Used the correct format: [List Name](link) - Description
• Checked spelling and grammar