documentation: Add horizontal scrollbar to code blocks in help center.

Some code blocks in the help center have lines that are too long
to fit, and wrap onto the next line. This can look awkward, and
may cause confusion. An horizontal scroll bar is added to the code
blocks to help fit everything into their own lines and clear up
confusions.

Fixes #24004.

web/styles/portico/markdown.css

@@ -143,6 +143,11 @@
 
             .codehilite {
                 background-color: hsl(0, 0%, 100%);
+
+                pre {
+                    white-space: pre;
+                    overflow-x: auto;
+                }
             }
 
             & > ul {