Split galerie's second section into title + text fields

- Add header.section2_title (CMS field: "Titel - rechts beneden")
  alongside header.section2_text ("Tekst - rechts beneden")
- Render the title as its own heading above the text, instead of
  folding it into the markdown body
- Zero out the heading's top margin so it aligns with the top of the
  adjacent image grid

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

themes/natascha/css/custom.css

@@ -188,6 +188,10 @@ footer {
     margin-bottom: 0.75rem;
 }
 
+.content-text h2.section-subtitle {
+    margin-top: 0;
+}
+
 @media (min-width: 768px) {
     .content-text h2 {
         font-size: 1.7em;