Updated openpose sample to run using onnx

[gursimarsingh]

Pull Request Readiness Checklist

25006 #25314

openpose dnn sample has been renamed to human_pose and now supports onnx model. Steps for converting the model from caffe to onnx have been added in the description

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• [x] I agree to contribute to the project under Apache 2 License.
• [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• [x] The PR is proposed to the proper branch
• [x] There is a reference to the original bug report and related work
• [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name.
• [x] The feature is well documented and sample code can be built with the project CMake

[asmorkalov]

The core team decided not to drop Caffe support in 5.0, but mark it deprecated. I proposed to presume the original code unchanged. Closed for now.