Add `id` to all headings and link them from "Page Contents" in understanding docs

Particularly for long and heavily structured understanding pages like https://www.w3.org/WAI/WCAG22/Understanding/non-text-contrast.html it would be nice to have a way to jump to/link directly to a particular section (like, say, https://www.w3.org/WAI/WCAG22/Understanding/non-text-contrast.html#boundaries).

Assume this would be straightforward to do with a bit of eleventy magic /cc @kfranqueiro 