Fix readthedocs.io documentation build
Since four days, readthedocs builds fail because of an error:
Could not import extension sphinx.builders.linkcheck (exception: urllib3 v2.0 only supports OpenSSL 1.1.1+)
The fix is mentioned in the GitHub issue: https://github.com/readthedocs/readthedocs.org/issues/10290
and implemented here by adding a new file called .readthedocs.yaml
.