doc: Add a doc_requirements.txt for ReadTheDocs
Created by: valschneider
Commit f3c2fbc1 ("bisector: add dbus dependencies") added an apt dependency that RtD cannot satisfy, so let's make a minimal requirements file just for doc building.
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Created by: valschneider
Commit f3c2fbc1 ("bisector: add dbus dependencies") added an apt dependency that RtD cannot satisfy, so let's make a minimal requirements file just for doc building.