ENH: Make sourcedir and outputdir parameters in sphinx-build optional

[timhoffm]

This allows to simply run sphinx-build in your doc directory, instead of having to type sphinx-build . _build.

The default output folder name "_build" is a bit opinionated, but it's what sphinx-quickstart uses. It creates that folder and adds it to exclude_patterns in conf.py already. I believe, we can therefore afford to go with this default. One may later add an output_dir config variable to conf.py and use that instead of a hard-coded default.