Add Two Pointers technique documentation

[Copilot]

Adds comprehensive documentation for Two Pointers patterns (opposite direction and same direction) following the structure established in SLIDING_WINDOW.md.

Changes

New Documentation: docs/techniques/TWO_POINTERS.md

• Opposite Direction (Convergent): Pointers start at ends, converge toward middle
  • Examples: Two Sum II, Valid Palindrome, Container With Most Water, 3Sum
• Same Direction (Parallel): Pointers move forward at different speeds
  • Examples: Remove Duplicates, Move Zeroes, Partition Array
• Fast/Slow Variation: Cycle detection and finding middle elements
  • Examples: Floyd's algorithm, linked list middle

Each pattern includes:

• Reusable code templates with complexity analysis
• Flow diagrams (Mermaid)
• Common pitfalls with wrong vs correct implementations
• 15+ categorized LeetCode problems

Updated: docs/README.md

• Added Two Pointers entry with pattern summary

Example Template

// Opposite Direction Pattern
const oppositeDirectionPattern = (arr) => {
    let left = 0;
    let right = arr.length - 1;
    
    while (left < right) {
        if (/* condition to move left */) {
            left++;
        } else if (/* condition to move right */) {
            right--;
        } else {
            left++;
            right--;
        }
    }
    return result;
};

Pro tip: If problem mentions sorted array with pairs/triplets → use opposite direction. If in-place modification needed → use same direction.

Original prompt

Add documentation to the docs/techniques for two pointers - both opposite and same direction techniques. Look at the docs/techniques/SLIDING_WINDOW.md for reference on current documentation

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