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>

src/primitives/icon.rs

@@ -226,6 +226,7 @@ icons! {
     Merge => "merge",
     MessageSquare => "message-square",
     MessageSquareDiff => "message-square-diff",
+    MessageSquareWarning => "message-square-warning",
     MicOff => "mic-off",
     Mic => "mic",
     Minimize2 => "minimize-2",