doc: create a comprehensive architectural diagram of the purecv codebase

[kalwalt]

Description

To improve onboarding and codebase understanding, we should create a comprehensive, high-level architecture diagram representing the entire purecv project structure, dependencies, and internal module relationships.

Scope of the Diagram

The diagram should capture:

1. Core Crate Layer (lib.rs): Re-exports, the public prelude, and module configurations.
2. core Module:
   • matrix.rs (The central Matrix<T> structure)
   • arithm.rs (Arithmetic, linear algebra, and element-wise matrix operations)
   • types.rs (Point, Size, Rect, Scalar, etc.)
   • structural.rs (Transpositions, flips, rotations, splitting, merging)
   • Utility structures, borders, and error handling (error.rs, utils.rs).
3. imgproc Module:
   • Color space conversions (color.rs)
   • Filtering algorithms (filter.rs - blur, bilateral, box, gaussian, median)
   • Edge and derivative computation (edge.rs, derivatives.rs - Sobel, Scharr, Laplacian, Canny)
   • Thresholding (threshold.rs)
4. features2d Module:
   • Keypoints (keypoint.rs)
   • Feature detectors (fast.rs, orb.rs)

Suggested Visual Format

A structured Mermaid graph (flowchart or class/dependency diagram) integrated directly into the README.md or a new design systems folder, using clear grouping (subgraphs) for each main package directory (core, imgproc, features2d).

Additional Goals

• Embed the generated Mermaid diagram in the main README.md and/or src/lib.rs package-level documentation.
• Maintain consistency in styling (e.g., color-coding subgraphs for core modules vs. image processing algorithms).