docs: fix build without opencv_contrib (avoid links to CUDA modules)

doc/tutorials/gpu/gpu-basics-similarity/gpu_basics_similarity.markdown

@@ -1,3 +1,4 @@
+@cond CUDA_MODULES
 Similarity check (PNSR and SSIM) on the GPU {#tutorial_gpu_basics_similarity}
 ===========================================
 @todo update this tutorial
@@ -204,3 +205,4 @@ It may be just the improvement needed for your application to work. You may obse
 instance of this on the [YouTube here](https://www.youtube.com/watch?v=3_ESXmFlnvY).
 
 @youtube{3_ESXmFlnvY}
+@endcond

doc/tutorials/gpu/gpu-thrust-interop/gpu_thrust_interop.markdown

@@ -1,3 +1,4 @@
+@cond CUDA_MODULES
 Using a cv::cuda::GpuMat with thrust {#tutorial_gpu_thrust_interop}
 ===========================================
 
@@ -68,3 +69,4 @@ Next we will determine how many values are greater than 0 by using thrust::count
 
 We will use those results to create an output buffer for storing the copied values, we will then use copy_if with the same predicate to populate the output buffer.
 Lastly we will download the values into a CPU mat for viewing.
+@endcond

doc/tutorials/gpu/table_of_content_gpu.markdown

@@ -1,3 +1,4 @@
+@cond CUDA_MODULES
 GPU-Accelerated Computer Vision (cuda module) {#tutorial_table_of_content_gpu}
 =============================================
 
@@ -20,3 +21,4 @@ run the OpenCV algorithms.
 
     This tutorial will show you how to wrap a GpuMat into a thrust iterator in order to be able to
     use the functions in the thrust library.
+@endcond

doc/tutorials/introduction/transition_guide/transition_guide.markdown

@@ -236,7 +236,10 @@ for(;;){
 
 CUDA {#tutorial_transition_hints_cuda}
 ----
-_cuda_ module has been split into several smaller pieces:
+
+CUDA modules has been moved into opencv_contrib repository.
+
+@cond CUDA_MODULES
 - _cuda_ - @ref cuda
 - _cudaarithm_ - @ref cudaarithm
 - _cudabgsegm_ - @ref cudabgsegm
@@ -249,10 +252,7 @@ _cuda_ module has been split into several smaller pieces:
 - _cudastereo_ - @ref cudastereo
 - _cudawarping_ - @ref cudawarping
 - _cudev_ - @ref cudev
-
-`gpu` namespace has been removed, use cv::cuda namespace instead. Many classes has also been renamed, for example:
-- `gpu::FAST_GPU` -> cv::cuda::FastFeatureDetector
-- `gpu::createBoxFilter_GPU` -> cv::cuda::createBoxFilter
+@endcond
 
 Documentation format {#tutorial_transition_docs}
 --------------------

doc/tutorials/tutorials.markdown

@@ -76,11 +76,13 @@ As always, we would be happy to hear your comments and receive your contribution
 
     Learn how to create beautiful photo panoramas and more with OpenCV stitching pipeline.
 
+@cond CUDA_MODULES
 -   @subpage tutorial_table_of_content_gpu
 
     Squeeze out every
     little computational power from your system by utilizing the power of your video card to run the
     OpenCV algorithms.
+@endcond
 
 -   @subpage tutorial_table_of_content_ios