Solving sudoku using Backtracking challenge snippet

Added the code snippet for solving a sudoku using backtracking. I have given a short description of the algorithm and links to various sources for those interested in learning more about backtracking.

Fixes: #13

Type of change

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• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generates no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules