[DOC] Fix broken "Contributors" link in example notebooks

[kabirvashisht4-glitch]

Describe the documentation issue

The 'Contributors' link found at the bottom of the example notebooks in the examples/ directory currently points to CONTRIBUTORS.md. This file does not exist in the repository, resulting in a 404 error. The link should be updated to point to the existing AUTHORS.rst file to properly attribute contributors.

Location
All .ipynb files within the examples/ folder

Solution
Current (Broken): [Contributors] - https://github.com/sktime/skpro/blob/main/CONTRIBUTORS.md

Corrected: [Contributors] - https://github.com/sktime/skpro/blob/main/AUTHORS.rst

[kabirvashisht4-glitch]

@fkiraly, could you please review this Issue