Add anchor links, attempt #1.

layouts/_default/_markup/render-heading.html

@@ -0,0 +1,26 @@
+<!--
+	For now, make the anchor link invisible to screen readers.
+		1. The anchor would need an accessible name, but aria-labels don't
+		   translate well with machine translation. It would need to be
+			 labelledby another element hidden by CSS, which would crowd the
+			 page for non-CSS users.
+		2. Screen reader users don't depend on visually identifying heading
+		   levels and can easily jump between headings without scrolling to
+			 the table of contents, so these anchor links are slightly less
+			 useful to them.
+		3. These links don't expose new functionality, since users could
+		   already copy heading anchor links in the TOC. This just makes some
+			 existing functionality slightly more easily. So it's less of a big
+			 deal
+	Yeah I know, the situation isn't optimal.
+-->
+<h{{ .Level }} id="{{ .Anchor | safeURL }}">{{ .Text | safeHTML }}
+{{ if gt .Level 1 -}}
+	<a href="#{{ .Anchor | safeURL }}" aria-hidden="true" tabindex="-1">
+		{{- $octothorpes := sub .Level 1 -}}
+		{{- range $i, $sequence := (seq $octothorpes) -}}
+		{{- print `#` -}}
+		{{- end -}}
+	</a>
+{{- end }}
+</h{{ .Level }}>