Make callout headings explicit

scripts/import-quarto-blog.mjs

@@ -279,7 +279,8 @@ authorName: "Neeldhara"
 - Standard Markdown headings, lists, links, tables, code fences, footnotes, and raw HTML are preserved.
 - Use colocated relative assets: \`![Caption](image.png)\`.
 - Quarto image attributes such as \`{width=70%}\` are removed during import. Use normal Markdown now; layout should be handled by Astro CSS.
-- Quarto callouts are converted to simple blockquotes:
+- Quarto callouts are converted to simple blockquotes. The callout type controls
+  styling only; it does not create a visible heading by default:
 
 \`\`\`markdown
 > **Note**
@@ -287,14 +288,22 @@ authorName: "Neeldhara"
 > Text of the callout.
 \`\`\`
 
-- For new notes, use the same blockquote convention for portable callouts:
+- For new notes, the bracket form is also supported:
 
 \`\`\`markdown
-> **Warning**
+> [!warning]
 >
 > This point needs attention.
 \`\`\`
 
+- To show a heading, put it explicitly inside the bracket after \`:\` or \`|\`:
+
+\`\`\`markdown
+> [!tip: Useful observation]
+>
+> Text of the callout.
+\`\`\`
+
 - Quarto margin notes such as \`[text]{.aside}\` are converted to:
 
 \`\`\`markdown