docs: Add mathematical proof for Lagrange interpolation optimizations

[GarmashAlex]

Description

This PR adds a comprehensive mathematical proof for two key optimization points in the Lagrange interpolator implementation:

1. The formula v_inv[0] = m*h^{m-1} - Proven by analyzing the product representation and leveraging properties of multiplicative groups
2. The recurrence relation v_inv[i] = g^{-1} * v_inv[i-1] - Derived through algebraic manipulation and group theory properties

These optimizations significantly reduce the computational cost of Lagrange interpolation by avoiding redundant calculations and leveraging mathematical properties of finite fields.

closes: #XXXX

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• Wrote unit tests (N/A - Documentation