Use the new method to set canonical URL for RTD
ReadTheDocs is deprecating Sphinx context injection which is used (among other things) for:
Setting the canonical URL based on the domain defined in Read the Docs.
https://about.readthedocs.com/blog/2024/07/addons-by-default/
This PR implements the recommended way (https://about.readthedocs.com/blog/2024/07/addons-by-default/#how-does-it-affect-my-projects)