Add Explanations for Missing Problems

Add explain.md for Pythagorean Triplet - MEDIUM

**Description:**
- Problem: https://www.geeksforgeeks.org/problems/pythagorean-triplet3018/1
- Difficulty: Medium
- Status: Solutions exist but explanation is missing
- What's needed: explain.md with:
  - Problem statement
  - Approach & intuition
  - Complexity analysis
  - Example walkthrough