[Infrastructure Feature Request] Conditionally add heading numbering

[hamishwillee]

Section headings and nesting are generally easily inferred from heading height. In #2021 a user reported that they have difficulty with distinguishing the sections and would like heading numbers.

We might in future conditionally add CSS to add these for users that want them - i.e. using https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Counter_Styles/Using_CSS_counters

Note that I think the site is sufficiently accessible for the vast number of users, and I wouldn't go to the effort of adding this at the cost of complicating the UI for most people. However if we can help people who want this feature, it is akin to providing a dark mode - a "nice to have".