doc: sphinx master_doc: set to index explicitly
doc: sphinx master_doc: set to index explicitly

https://www.sphinx-doc.org/en/master/usage/configuration.html?highlight=master_doc#confval-master_doc
master document name was 'contents' before 2.0, set to 'index' always

certtools/intelmq,certtools/intelmq,certtools/intelmq,aaronkaplan/intelmq,aaronkaplan/intelmq,aaronkaplan/intelmq