Add article heading anchors

Adds heading anchors (#) that appear when user hovers over headings in
articles. Clicking the anchor will scroll viewport until heading is
aligned to top and update the address bar with the hash. This makes it
easy to copy URLs that link to a specific heading in the article.

assets/css/main.css

@@ -37,6 +37,19 @@ h6 {
   margin-bottom: var(--hx-margin-bottom);
 }
 
+.heading {
+  a {
+    text-decoration: none;
+    color: var(--content-secondary);
+    visibility: hidden;
+    font-size: 0.95em;
+  }
+
+  &:hover a {
+    visibility: visible;
+  }
+}
+
 p {
   margin-top: var(--p-margin-top);
   margin-bottom: var(--p-margin-bottom);
@@ -527,4 +540,4 @@ blockquote {
 blockquote p {
   margin-left: 1rem;
   margin-right: 1rem;
-}
+}

layouts/_default/_markup/render-heading.html

@@ -0,0 +1,4 @@
+<h{{ .Level }} class="heading" id="{{ .Anchor }}">
+  {{ .Text }}
+  <a href="#{{ .Anchor }}">#</a>
+</h{{ .Level }}>