docs: contain image sizing and prevent overflow on mobile

`max-width: 100%` keeps images from expanding beyond
their container and creating horizontal overflow scroll
on small screens.

`height: auto` ensures the image scales proportionally
when width is adjusted.

docs/style.css

@@ -116,6 +116,10 @@ a {
 a:hover {
   text-decoration: underline;
 }
+img {
+  max-width: 100%;
+  height: auto;
+}
 b {
   font-weight: 600;
 }