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/templates/galerie.html.twig

@@ -47,6 +47,9 @@
 
 <div class="row g-3 mt-2">
     <div class="col-12 col-md-6 p-3 text-start content-text order-2">
+        {% if page.header.section2_title %}
+        <h2 class="section-subtitle">{{ page.header.section2_title }}</h2>
+        {% endif %}
         {{ page.header.section2_text|markdown }}
         <div class="d-flex flex-column flex-sm-row gap-2 mt-3">
             <a href="{{ url('/werk-in-detail') }}" class="btn btn-brand">{{ is_en ? 'View work in detail' : 'Bekijk werk in detail' }}</a>