Add Antora's "Content" TOC to the preview

[ahus1]

Why the new feature should be added

On the rendered Antora website, each page has a "Content" TOC, which is currently not shown in the preview. For users who expect the preview to match the page, this comes as a surprise.

See https://antora.zulipchat.com/#narrow/stream/282400-users/topic/Intelli.20.22.3Atoc.3A.20left.22.20does.20not.20render.20the.20toc.20to.20the.20left.20in

How the new feature should work

This feature might be optional. On a smaller screen (less width) the contents could be collapsed -- although this would deviate from the preview of the real site.

[mojavelinux]

If this is added, I strongly suggest tucking it away in either a dropdown or hamburger icon. I don't think the TOC should be shown in a separate column in a preview. That is just a tremendous waste of space that severally reduces the amount of content that can be previewed.

although this would deviate from the preview of the real site.

I don't think the preview should be emulating exactly what's in the real site. That's just not the point of a preview...and it will never be able to match completely anyway without rendering the entire site, which would just be such a waste.