feat(website): improve css for markdown pages (#3129)

- Give h3 proper top margin (and also add h4 just in case)
- Give images some y margin

Relates to pathoplexus/pathoplexus#262
See https://loculus.slack.com/archives/C05GWA22R19/p1730378998996159?thread_ts=1730377039.594559&cid=C05GWA22R19

website/src/styles/mdcontainer.scss

@@ -12,9 +12,17 @@
   }
 
   .mdContainer h3{
-    @apply text-lg font-semibold text-main my-3;
+    @apply text-lg font-semibold text-main my-3 mt-5;
 
   }
+
+.mdContainer h4{
+    @apply text-base font-semibold text-main my-3 mt-4;
+  }
+
+.mdContainer img{
+    @apply my-3;
+  }
 
   .mdContainer p{
     @apply my-2;