Merge pull request #26469 from vpisarev:move_gapi_to_contrib_part1

Removed g-api from the main repo #26469

Following #25000.
CI patch: https://github.com/opencv/ci-gha-workflow/pull/196

This is migration of G-API from opencv to opencv_contrib, part 1.
Here we simply remove G-API from the main repo. The next patch should bring G-API to opencv_contrib.

- [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.
- [ ] The feature is well documented and sample code can be built with the project CMake

doc/tutorials/tutorials.markdown

@@ -9,7 +9,6 @@ OpenCV Tutorials {#tutorial_root}
 - @subpage tutorial_table_of_content_objdetect - INSERT OBJDETECT MODULE INFO
 - @subpage tutorial_table_of_content_features - feature detectors, descriptors and matching framework
 - @subpage tutorial_table_of_content_dnn - infer neural networks using built-in _dnn_ module
-- @subpage tutorial_table_of_content_gapi - graph-based approach to computer vision algorithms building
 - @subpage tutorial_table_of_content_other - other modules (stitching, video, photo)
 - @subpage tutorial_table_of_content_ios - running OpenCV on an iDevice
 - @subpage tutorial_table_of_content_3d - 3d objects processing and visualisation