Add Doxygen documentation across traincascade and refresh project READMEs

[vladiant]

Summary

Documents the cascade-trainer library, the traincascade CLI entry point and the test suite with Doxygen-style comments, and rewrites the two top-level README files so the repository structure and build flow are discoverable. No functional code changes — only comments and Markdown.

Motivation

The traincascade sources were largely undocumented and the project README contained only a one-line description. New contributors had to read the OpenCV legacy ML hierarchy (CvStatModel → CvDTree/CvBoost → cascade subclasses) end-to-end to understand the layout. This MR adds inline API documentation and a high-level map so the codebase is self-describing.

Changes

Doxygen comments — public headers (include)

• File-level @file briefs on all 20 headers.
• Class/struct briefs and field-level docs on parameter types: CvCascadeParams, CvCascadeBoostParams, CvDTreeParams, CvBoostParams, CvHaarFeatureParams, CvLBPFeatureParams, CvHOGFeatureParams.
• Method-level @brief / @param / @return on the main entry points: CvCascadeClassifier::train, the CvFeatureEvaluator interface, CvDTree's train/predict/find_split_*/prune_cv, CvBoost/CvCascadeBoost/CvCascadeBoostTree, and the CvDTreeTrainData / CvCascadeBoostTrainData setup helpers.
• Documented the CC_* XML tag macros and the CV_SUM_OFFSETS / CV_TILTED_OFFSETS integral-image helpers.

Doxygen comments — driver

• traincascade.cpp: added @file block, @brief on main(), and per-variable doc comments explaining the parameter structs and the argument-parsing fallthrough into scanAttr().

Markdown

• README.md: expanded "About" with project purpose, repository-layout table, build instructions (CMake + Ninja, coverage build), architecture overview of the class hierarchy, doc links, and a license note distinguishing OpenCV-derived files. CI badges preserved.
• README.md: added a per-file table mapping each test_*.cpp to the component it exercises, plus CTest run instructions and a pointer to the coverage build.

Intentionally not modified

OpenCV-licensed verbatim sources keep their original Intel / OpenCV Foundation headers untouched:

• traincascade/lib/src/o_*.cpp
• annotation, createsamples, visualisation

Validation

• cmake --build build --target traincascade → [10/10] Linking CXX executable traincascade/traincascade, clean.
• No new diagnostics on edited headers.
• Diff: 23 files, +934 / −106 (comments and Markdown only).

Checklist

• Build succeeds (Ninja).
• No source/behavior changes — comments and docs only.
• CI badges and license headers preserved.
• Test-suite layout documented.