Expose Sphinx option to treat warnings as errors

[oggy-]

sphinx-build -W turns warnings into errors, but this option is not currently exposed by sbt-site.

[lightbend-cla-validator]

Hi @oggy-,

Thank you for your contribution! We really value the time you've taken to put this together.

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[ennru]

Hi @oggy- In case you are ready to help, I'm trying to bring some life back to sbt-site, but I didn't invest time in bringing the Sphinx tests back. See #186