Error
{#
I'd like to use ``build.error|urlize`` here, so we can have nice links.
However, this is not possible because we are using `data-bind="text: error"`
which means that Knockout.js will use the `.error` attribute to fill
the content of this tag dynamically.
#}
The configuration key "build.os" is required to build your documentation. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os
