Bump header level styles in sidebar if needed

[seshrs]

Currently, Primer Spec adds a little border for smaller headings underneath a H1. However, if the top-level heading is a H2, the style doesn’t appear.

I think it makes sense to bump all header level styles in the sidebar so that the highest-found header level is styled as a H1.

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Tangent: I wonder if we should also auto-ignore the first title H1 of the page from the sidebar. Or maybe it should be presented differently in the sidebar? I think I want to explore alternative header designs a bit more.

But maybe we should provide an option to

[awdeorio]

Tangent: I wonder if we should also auto-ignore the first title H1 of the page from the sidebar. Or maybe it should be presented differently in the sidebar? I think I want to explore alternative header designs a bit more.

I think this idea makes sense. Typically, a document with have an H1 (the title). All sections of the doc will be H2 and lower. Usually, the title of a document does not appear in a table of contents. Perhaps default to hiding the first H1, with an option to show?