Add lesson: Merge Sort

This PR adds a complete lesson for Merge Sort, including:

• Introduction and explanation
• Step-by-step illustration
• Java code implementation
• Complexity analysis and stability
• Applications

Closes #74

Fixes #

Short description of what this resolves:

What Changes proposed in this pull request:

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added code examples, test must pass
• Added documentation so that the program is self-explanatory and educational.
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: