Specify master_doc to avoid issues with older versions

[maximumcats]

On a system I was using where I was using sphinx and sphinx-rtd-theme from Ubuntu rather than from pip, my version of sphinx-build was old enough (1.6.7) that the RTD theme didn't work without specifying master_doc. I found the solution on SO:

https://stackoverflow.com/questions/56336234/build-fail-sphinx-error-contents-rst-not-found

which is consistent with the Sphinx documentation:

https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-master_doc

I tested this on both my older install that required it, and a newer one that didn't, and it works for both cases.