use root-relative URLs in margin-header

so that it will work for documents in subdirs, too.
Quarto will transform these correctly when rendering,
so that they'll work even when the project isn't served at a server root.

_quarto.yml

@@ -4,8 +4,8 @@ project:
 website:
   title: "dbs-reference"
   margin-header: |
-    ![Logo image OST](assets/ost_logo.png)
-    ![Logo image IFS](assets/ifs_logo.png)
+    ![Logo image OST](/assets/ost_logo.png)
+    ![Logo image IFS](/assets/ifs_logo.png)
   navbar:
     left:
       - href: index.qmd