:beetle: Fix img-fluid in content generated from markdown

Was not working by setting p parent because pandoc encapsulate imgs in
figures.

So the parent to set is figure.

Beneficial side-effect : this rule target only markdown generated
content. No more need of root class generated-body.

src/include/css/custom/main.css

@@ -150,7 +150,7 @@ table tbody + tbody {
     border-top: 2px solid #dee2e6;
 }
 
-.generated-body p > img { /* img-fluid */
+figure > img { /* img-fluid */
     max-width: 100%;
     height: auto;
 }