Add Cholesky decomposition implementation

[markonnn]

Feature description

Proposal: Add an implementation of the Cholesky factorization for symmetric, positive-definite matrices within the linear_algebra module.

The module currently lacks a Cholesky factorization. Introducing this implemntation will make the underlying algorithmic steps explicit and enhance educational clarity.

My plan:

• Implement cholesky_decomposition(A) -> L
• Implement cholesky_solve(A, b) using forward/back substitution
• Add full docstrings with doctests
• Add pytest tests in the test directory
• Include validation for symmetry and positive-definiteness where appropriate
• Follow the folder structure and style of the existing algorithms

Please indicate whether this contribution is appropriate or if there are any specific guidelines I should follow.