Limit all headings to 40rem, same as paragraph text

We've received complaints that headings take up more horizontal width than text in the documentation layout, where text is not constrained to narrow columns, but spans the full available width until it hits the 40em max-width. For bigger font-size headings, that's a lot wider than surronding paragraphs. I propose we limit everything to 40rem as opposed to 40em, so things look tidier.

Start by locating the styles that set the maximum width for documentation paragraphs and headings, then compare their current em-based behavior with the proposed 40rem limit. Confirm the change across the documentation layout at larger heading sizes; done means headings and surrounding paragraph text stop at the same 40rem width.

Written by the indexing model from the issue text.