SEP 30: Adjust site style to permit offsetting

I find the sidebar table-of-contents at peps.python.org particularly pleasing.

I currently have a somewhat similar design for pages that make use of a TOC, but it is made slightly awkward by rigidly centring the main content. I would like for the main right margin to collapse first to maintain space on the left for the sidebar.

The python.org sidebar as seen beside a PEP My current, primitive sidebar