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/blueprints/galerie.yaml

@@ -33,9 +33,12 @@ form:
                   label: Foto
                   preview_images: true
 
+            header.section2_title:
+              type: text
+              label: 'Titel – rechts beneden'
             header.section2_text:
               type: markdown
-              label: 'Tekst – sectie 2 (onder)'
+              label: 'Tekst – rechts beneden'
               validate:
                 type: textarea
             header.section2_images: