Fix mkdocs build integration with readthedocs

[jmiranda]

I don't think much about this integration because it always just works, but I seem to have ignored the warning from months ago.
https://readthedocs.org/projects/openboxes/builds/22279630/

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"build.image" config key is deprecated and it will be removed soon. Read our blog post to know how to migrate to new key "build.os" and ensure your project continues building successfully.

Error
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