…dation

Updated trackbar callback function and improved documentation opencv#26524

This Fixes opencv#26467

Description:
This pull request improve the OpenCV documentation regarding the Trackbar functionality. The current documentation does not provide clear guidance on certain aspects, such as handling the value pointer deprecation and utilizing callback arguments in C. This update addresses those gaps and provides an updated example for better clarity.

Changes:
Updated Documentation:

Clarified the usage of the value pointer and explained how to pass an initial value, since the value pointer is deprecated.
Added more detailed explanations about callback arguments in C, ensuring that users understand how to access and use them in Trackbar callbacks.
Added a note on how to properly handle initial value passing without relying on the deprecated value pointer.
Updated Tutorial Example:

Renamed and used callback function parameters to make them more understandable.
Included a demonstration on how to utilize userdata in the callback function.
Additional Notes:

Removed reliance on the value pointer for updating trackbar values. Users are now encouraged to use other mechanisms as per the current implementation to avoid the runtime warning.

### Pull Request Readiness Checklist

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
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [] The feature is well documented and sample code can be built with the project CMake