fix: add build.os to fix the failing docs build

[iamsobanjaved]

Docs build has been failing for the last 4 months. These build details are necessary.

https://readthedocs.org/projects/open-edx-devstack/builds/?page=3

RTD Docs: https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

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I've completed each of the following or determined they are not applicable:

• [ ] Made a plan to communicate any major developer interface changes (or N/A)