.readthedocs.yaml is how the file is mentioned in the docs. Adjust the
name.

The latest Sphinx versions require at least 3.9.

Docs builds currently fail with

Could not import extension sphinx.builders.linkcheck
(exception: urllib3 v2.0 only supports OpenSSL 1.1.1+,
currently the 'ssl' module is compiled with OpenSSL
1.0.2n  7 Dec 2017.
See: https://github.com/urllib3/urllib3/issues/2168)

The Gluon issuetracker suggests specifying the build-container.

Link: https://github.com/readthedocs/readthedocs.org/issues/10290#issuecomment-1535120995



Signed-off-by: David Bauer <mail@david-bauer.net>