:art: Improve figure style from markdown

Pandoc encapsulate images into figures.

Add CSS to center image et colorize caption.

src/include/css/custom/main.css

@@ -150,7 +150,15 @@ table tbody + tbody {
     border-top: 2px solid #dee2e6;
 }
 
+figure {
+    text-align: center;
+}
+
 figure > img { /* img-fluid */
     max-width: 100%;
     height: auto;
 }
+
+figcaption {
+    color: #5e5e5e;
+}