Add CellularLab: iPerf3 + JNI + Gemini AI Android example

[Abhi5h3k]

This PR adds CellularLab to the Code examples section.

It's an open-source Android project demonstrating how to:

• Use CMake + JNI to integrate native C/C++ libraries (iPerf3)
• Build a Kotlin-based UI for network diagnostics
• Use Gemini AI to analyze performance logs and generate structured markdown reports

Useful for devs exploring native Android interop, network testing tools, or AI-powered UX features.