Add Armstrong Number algorithm with comprehensive tests

[duvvuvenkataramana]

Description

This PR adds an Armstrong Number (Narcissistic Number) algorithm implementation with comprehensive unit tests.

Changes Made

• ✅ Implemented ArmstrongNumber.isArmstrong() method
• ✅ Added detailed JavaDoc documentation with examples
• ✅ Created comprehensive unit tests covering:
  • Known Armstrong numbers (0, 1, 153, 370, 371, 407, 1634)
  • Non-Armstrong numbers (10, 100, 152, 200)
  • Edge cases (negative numbers)
• ✅ Follows project coding standards and package structure

Algorithm Explanation

An Armstrong number equals the sum of its digits each raised to the power of the number of digits.

Example: 153 = 1³ + 5³ + 3³ = 1 + 125 + 27 = 153

File Structure

• src/main/java/com/thealgorithms/maths/ArmstrongNumber.java - Main algorithm
• src/test/java/com/thealgorithms/maths/ArmstrongNumberTest.java - Unit tests

Type of Change

• [x] New algorithm implementation
• [x] Test cases added