Update mergesort.md

[chanichanichanichani]

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: