Merge pull request #29173 from s-trinh:add_parallel_for_tuto_link

Add parallel_for_ tutorial (Mandelbrot set) link in the main page #29173

Add the `parallel_for_` tutorial with Mandelbrot set use case in the main page:
- the tutorial exists: https://docs.opencv.org/4.13.0/d7/dff/tutorial_how_to_use_OpenCV_parallel_for_.html
- but is not listed in the main page: https://docs.opencv.org/4.13.0/de/d7a/tutorial_table_of_content_core.html

Some minor cleaning:
- removed [C=](https://www.hoopoesnest.com/cstripes/cstripes-sketch.htm) mention
- use Web Archive link
- prefer https link

On my computer (Firefox), the images ([here](https://docs.opencv.org/4.13.0/d3/dc1/tutorial_basic_linear_transform.html)) are shown like this:

<img width="1843" height="906" alt="image" src="https://github.com/user-attachments/assets/bc8386f5-8ea6-4fb7-92d1-75d72b0b9408" />

When adding a scale parameter, the images are correctly shown:

<img width="1445" height="873" alt="image" src="https://github.com/user-attachments/assets/e163a8e5-be0f-4139-aa51-b465fd619dc9" />

This is odd.

### 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
- [ ] 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/core/adding_images/adding_images.markdown

@@ -25,7 +25,7 @@ Theory
 
 @note
    The explanation below belongs to the book [Computer Vision: Algorithms and
-    Applications](http://szeliski.org/Book/) by Richard Szeliski
+    Applications](https://szeliski.org/Book/) by Richard Szeliski
 
 From our previous tutorial, we already know a bit of *Pixel operators*. An interesting dyadic
 (two-input) operator is the *linear blend operator*: