Make main content div wider

Looking at the theme now, with some more content, including example code, it would be nice if the main content section were a bit wider by default. We are doing a pretty good job with keeping our code within 80 characters, but we are still seeing a lot cut off.

Best place to look is probably: https://scpca.readthedocs.io/en/latest/faq.html#what-if-i-want-to-use-seurat-instead-of-bioconductor