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/documents/document/visual.rs

@@ -101,6 +101,7 @@ impl MarkdownDocument {
                     nodes,
                     &self.base,
                     &self.on_link,
+                    &self.on_heading,
                     self.look.scale,
                     cx,
                 )),