Table of contents and about page rework

[vincerubinetti]

• simplify flex component props, e.g. direction="column" -> column, since it's used everywhere. find+replace whole app.
• change heading component to register/unregister itself in global list of headings
• consistent use of data attribute for active state styles
• tweak table of contents styles
• instead of requerying for all headings on any document mutation (expensive*), render table of contents from global reactive list of headings
• include icons in table of contents entries
• clean up and compactify about page (original impetus for making TOC look and behave better)

* On any modern machine though, probably not a humanly perceptible difference. Also the expense will be limited by the fact that we should never have as many things on one page as is currently on the testbed page.

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[falquaddoomi]

I gave the code a quick look; the code changes make sense as far as I can tell, and seem cleaner.

IMO the about page looks way more organized than before. I like the new TOC styling, and especially the icons in it for each header. Nice work!