Enclose scribble parts in an html section tag

[stevebyan]

Enhance the Scribble html renderer to enclose the @title, @section, @subsection, etc parts in an html <section> tag. This enables better support for assistive technologies, and some CSS layout strategies such as a "block-based" design. For the latter, see Shriram Krishnamurthi's thread on the Racket Discord.

The section tag class attribute is set to the level of the part in the of SsectionLevel<n>, and the id attribute is set to section <the section number>. The id attribute is necessary for "block-based" design.

[stevebyan]

This has been only lightly tested. Volunteers welcome.

I should add the new class names to the Scribble docs for the CSS classes. I don't see any place in the current documentation where it would be appropriate to document the format of the id attribute for the <section> tag. Suggestions would be welcome.

[stevebyan]

Oops, looks like I have some more work to do, some tests are failing.

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[mflatt]

One thought/question: The CSS class name associated with a section tag in this PR is based on the absolute level of the section within the document, as opposed to the relative level on a rendered page. For example, the header at the top of https://docs.racket-lang.org/reference/eval-model.html is at level 2, while the header at the top of https://docs.racket-lang.org/pkg/getting-started.html is at level 1, but those render the same (with h2, currently) because they're at the level relative to the page. Would the class be more useful for styling if it were relative to the output page? Or maybe both an absolute class name and a relative class name would be useful?

[stevebyan]

@mflatt that's a good point. How might I implement relative levels, just subtract part-nesting-depth from number-depth?

Since the class is purely for page layout, not sematics, it makes sense to me to just have relative levels.