_content/css: set img max width to 100%

Fixes golang/go#65139

Change-Id: I9284fb5d2aae5f4dcff2e422f163381aa19fdb09
Reviewed-on: https://go-review.googlesource.com/c/website/+/556416
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: hyangah

_content/css/styles.css

@@ -869,6 +869,9 @@ h1 + h2.subtitle {
   padding: 0.5rem 1.75rem;
   border-radius: 4px;
 }
+.Article img {
+  max-width: 100%;
+}
 .CopyPaste {
   display: flex;
   align-items: center;