Add sphinx configuration key for readthedocs

[timwoj]

RTD started enforcing the existence of this key in their yml file as of 2025/01/20. See https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/ for more info. https://readthedocs.org/projects/zeek-docs/builds/26952573/ is a build that includes the config option for testing.

Our master builds are currently failing due to the lack of this key.

[awelzel]

Spicy has the key already, Broker does not. Any other readthedocs projects that may need a look?

[timwoj]

zkg is the only other one that uses readthedocs and it has it already. I'll open a PR on the broker side do a build there for sanity checking.