docs: Improve comments and documentation in Strassen algorithm

[ITZ-NIHALPATEL]

This pull request refactors and documents the implementation of Strassen's matrix multiplication algorithm in math/matrix/strassenmatrixmultiply.go. The changes improve code readability and maintainability by adding detailed comments, clarifying algorithmic steps, and grouping logic into themed sections. The function is now more understandable for future maintainers and learners.

Documentation and clarity improvements:

• Expanded the top-of-file comments to include a concrete example, a more precise explanation of Strassen's algorithm, and updated time complexity notation.
• Added detailed step-by-step comments within the StrassenMatrixMultiply function, labeling the main phases: divide, calculate intermediate terms, recursive multiplication, and combine results. [1] [2] [3]

Code organization and readability:

• Clarified creation and naming of submatrices and intermediate variables (e.g., A1, S1, M1) and annotated their mathematical meaning at each step.
• Added quadrant-specific comments when combining submatrices into the final result, making it easier to follow how the output matrix is constructed. [1] [2] [3]