Make width of collapsed <details> consistent

Previously the collapsed <details> would match the width of their
content, but this made them a little harder to recognize, and were
difficult to click on in (unusual) cases such as using a link as the
<summary>. Having them full-width like this should be fine with the
border, and fix those issues.

tildes/scss/_placeholders.scss

@@ -18,7 +18,7 @@
 
     &:not([open]) {
       cursor: pointer;
-      width: max-content;
+      max-width: $paragraph-max-width;
     }
   }