style: center all SVGs, images and GIFs in slides

Use display:block + auto margins to center visual elements
while keeping text left-aligned.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

index.html

@@ -45,6 +45,13 @@
       text-align: left;
     }
 
+    .reveal .slides section svg,
+    .reveal .slides section > img {
+      display: block;
+      margin-left: auto;
+      margin-right: auto;
+    }
+
     .reveal h1, .reveal h2 { text-align: left; }
 
     .title-slide, .title-slide h1 { text-align: center !important; }