DOCS: Move module descriptions from CONTRIBUTING table into module docs

The removed source layout table carried descriptions that existed nowhere
in rustdoc: complex and traits had no module docs at all, and several
first lines (Useful macros, Statistical Modules, Main structures) were too
vague to serve as an index. Each module now carries its one-line purpose
in its own mod.rs, so the docs.rs module index is the single source for
the source map and stays in sync with the code by construction.

Author: Axect

src/complex/mod.rs

@@ -1,3 +1,5 @@
+//! Complex vectors, matrices, and integrals, enabled by the `complex` feature
+
 use num_complex::Complex;
 
 pub type C64 = Complex<f64>;

src/macros/mod.rs

@@ -1,4 +1,4 @@
-//! Useful macros
+//! R / MATLAB / Julia style macros
 
 pub mod julia_macro;
 pub mod matlab_macro;

src/ml/mod.rs

@@ -1,3 +1,3 @@
-//! Machine learning tools
+//! Basic machine learning tools (beta)
 
 pub mod reg;

src/numerical/mod.rs

@@ -1,4 +1,4 @@
-//! Differential equations & Numerical Analysis tools
+//! Numerical algorithms: ODE solvers, quadrature, interpolation, splines, root finding, optimization, and eigenvalue computation
 
 pub mod eigen;
 pub mod integral;

src/statistics/mod.rs

@@ -1,4 +1,4 @@
-//! Statistical Modules
+//! Probability distributions, random sampling, and statistical operations
 //!
 //! * Basic statistical tools - `stat.rs`
 //! * Popular distributions - `dist.rs` (`rand` feature)

src/structure/mod.rs

@@ -1,8 +1,8 @@
-//! Main structures for peroxide
+//! Core data structures: `Matrix`, `Vec<f64>` extensions, `DataFrame`, `Polynomial`, and const-generic forward-mode AD (`Jet<N>`)
 //!
-//! * Matrix
+//! * Matrix (dense & sparse)
 //! * Vector
-//! * Automatic derivatives
+//! * Automatic differentiation (`Jet<N>`)
 //! * Polynomial
 //! * DataFrame
 //! * Multinomial (not yet implemented)

src/traits/mod.rs

@@ -1,3 +1,5 @@
+//! Shared trait definitions: mathematics, functional programming, mutability, pointers, and numeric abstractions
+
 pub mod float;
 pub mod fp;
 pub mod general;

src/util/mod.rs

@@ -1,4 +1,4 @@
-//! Utility - plot, print, pickle and etc.
+//! Utilities: constructors, printing, plotting, and low-level helpers
 
 pub mod api;
 pub mod non_macro;