Disable the "Back to top" button in the documentation

[hoechenberger]

Closes #12685

[cbrnr]

The docs build is failing. It would be good to see how the docs look like without that button though.

[larsoner]

It would be good to see how the docs look like without that button though.

The rendered docs are still available:

https://output.circle-artifacts.com/output/job/6b6ff620-ee6c-4671-ab7b-0f559f6dfecc/artifacts/0/html/index.html

When the build finishes with a Sphinx error the docs are usually still produced. (If it errors out before Sphinx gets started on the other hand, like during a download step, there are no artifacts to expose.)

[cbrnr]

Thanks @hoechenberger!