Added `title_image: default` on main pages

contact.md

 ---
 title: Contact
+title_image: default
 layout: default
 excerpt:
     Contact to HIFIS Office.

faq.md

 ---
 title: <i class="fas fa-question-circle"></i> FAQ - Frequently Asked Questions
+title_image: default
 layout: default
 excerpt:
     Collection of Frequently Asked Question (FAQ) about HIFIS.

imprint.md

 ---
 title: Imprint
+title_image: default
 layout: default
 ---
 {% if site.baseurl != '/' %}

privacy.md

 ---
 title: Privacy
+title_image: default
 layout: default
 ---
 
@@ -94,4 +95,4 @@ supervisory authority (Saxon data protection representative,
 at any time.
 
 In case of legal disputes, only the German version of this text is legally
-binding.
\ No newline at end of file
+binding.