[css-gaps-1] Add explainer and associated images

Author: kbabbitt

css-gaps-1/Overview.bs

@@ -998,7 +998,7 @@ Adjusting gap decoration endpoints: The 'rule-inset' properties</h3>
 			}
 		</pre>
 		<figure>
-			<img alt="" src="images/example-column-inset-minus-5px.png">
+			<img alt="" src="images/example-column-inset-5px.png">
 			<figcaption>
 				An inset of ''5px'' shortens the ends of <a>gap decorations</a>
 				relative to the edges of adjacent items.
@@ -1014,7 +1014,7 @@ Adjusting gap decoration endpoints: The 'rule-inset' properties</h3>
 			}
 		</pre>
 		<figure>
-			<img alt="" src="images/example-column-inset-5px.png">
+			<img alt="" src="images/example-column-inset-minus-5px.png">
 			<figcaption>
 				An inset of ''-5px'' extends the ends of <a>gap decorations</a>
 				slightly beyond the edges of adjacent items.