Render the table of contents, and group /blog/ by year

Two of the differences the page-by-page diff against the live site turned up.

The TOC was the larger one: live has one on 171 pages and the org-ssg build had
none. `[html] toc = true` only *exposes* `page.toc` — rendering it is the
layout's job, and base.html never did. It now emits the same
`div#table-of-contents` Emacs does, from a recursive macro over the tree. The
anchors already matched, so every link lands where the live one does. Pages that
say `#+OPTIONS: toc:nil` get an empty tree and render nothing, which is why
/uses/, /now/ and /salary/ have no TOC here either — the same three as live.

/blog/ now breaks its list under year headings, as the live index does, using
`page.year` and minijinja's groupby. `group_by_year` at the top of blog.html
turns it off for one flat list.

After both, /blog/ matches the live page's content exactly, and 171 of 171 TOCs
are accounted for.

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