features2d: write documentation and examples

[kalwalt]

Write top-level and item-level documentation for all public APIs added in the features2d module. Add usage examples in main crate docs and module-level docstrings. Provide Rust code examples for detection, description, and potential matching workflows (even if matcher is not implemented). Include links to OpenCV documentation for reference.

Example doc code:

use purecv::features2d::{FastFeatureDetector, FastType, Orb};
use purecv::core::Matrix;

let fast = FastFeatureDetector::new(20, true, FastType::Type9_16);
let corners = fast.detect(&image)?;

let orb = Orb::default();
let (keypoints, descriptors) = orb.detect_and_compute(&image)?;

References:

• https://docs.opencv.org/4.10.0/d5/d51/group__features2d__main.html