Added english tutorial for LOJ-1068 Investigation

[sandesh-543]

Closes #517.

Problem link: https://lightoj.com/problem/1068

Changes introduced in this PR

• This PR adds an English tutorial for problem 1068 – Investigation
• Explained the Digit DP technique with clear state tracking:
  • Current position
  • Remainder modulo K
  • Digit sum modulo K
  • Tight constraint flag
• Included optimized and memoized Java solution
• Provided input/output samples
• Added edge case handling notes for K > 90 and memory constraints
• Mentioned potential improvements to handle TLE and MLE on LightOJ