documents: GitHub alerts, a [toc] list of headings, front matter in a box, HTML blocks as text, a visible rule and bullets by depth

At the root a quote marked [!NOTE] and the rest, as GitHub's API reads it,
is an alert; a [toc] paragraph lists the headings and a press reveals one;
an HTML block shows its text with the inline tags, white space as a browser
lays it, scripts and styles dropped. The rule is 2px; bullets turn by depth.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

assets/icons/message-square-warning.svg

@@ -0,0 +1,17 @@
+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-message-square-warning"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M22 17a2 2 0 0 1-2 2H6.828a2 2 0 0 0-1.414.586l-2.202 2.202A.71.71 0 0 1 2 21.286V5a2 2 0 0 1 2-2h16a2 2 0 0 1 2 2z" />
+  <path d="M12 15h.01" />
+  <path d="M12 7v4" />
+</svg>