Offer anchors for the headings

While all of the headings already have an ID and therefore allow deep
links, it is cumbersome to figure out what the exact ID is.

Let's copy the trick I implemented in
git-for-windows/git-snapshots@3e10e48a and in
git-for-windows/git-snapshots@0dff3b9a: This
will show a clickable link symbol when hovering over a heading.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

assets/sass/style.scss

@@ -370,3 +370,11 @@ body.page {
 .markdown-important {
 	border-left: .25em solid #f34f29;
 }
+.anchor {
+	transform: translateX(-80%) scale(.6);
+	position: absolute;
+}
+
+:is(h1, h2, h3, h4):not(:hover) .anchor {
+	display: none;
+}

layouts/_default/single.html

@@ -1,3 +1,5 @@
 {{ define "main" }}
-{{ replace .Content "<p>[!IMPORTANT]" "<p class=\"markdown-important\">" | safeHTML }}
+{{ replace .Content "<p>[!IMPORTANT]" "<p class=\"markdown-important\">" |
+  replaceRE "(<h[1-4] id=\"([^\"]+)\"[^>]*>)" `${0}<a class="anchor" href="#${2}">&#128279;</a>` |
+  safeHTML }}
 {{ end }}