feat: Implement median function (closes #1)

[maitriupadhyay03-cell]

Overview

1. This PR fixes Implement median #1 (Implement median).
2. 2. This PR adds a median function to the primary module of the rustats crate. The function takes a slice of f64 values and returns Option<f64> — None for empty input, and Some(median) otherwise. It sorts a copy of the input and picks the middle element (or averages the two middle elements for even-length inputs).

Essential Checklist

• The PR title starts with "feat: Implement median function (closes Implement median #1)", followed by a short, clear summary of the changes.
• - [x] Tests pass: the existing tests/median.rs integration tests cover empty input, odd/even counts, sorted/unsorted input, negatives, decimals, and duplicates.
• - [x] Docs: the function includes rustdoc comments with an example.