Make the collapse/expand buttons in the new sidebar more accessible

They’re now marked as toggle buttons with `aria-pressed` and have better labels with `aria-label`.

Author: chasenlehara

sidebar/sidebar.stache

@@ -22,7 +22,11 @@
                 {{child.shortTitle}}
               </a>
               {{#if child.isCollapsible}}
-                <button on:click="child.collapse()">
+                <button
+                  aria-label="{{#if child.isCollapsed}}Expand{{else}}Collapse{{/if}} section"
+                  aria-pressed="{{#if child.isCollapsed}}false{{else}}true{{/if}}"
+                  on:click="child.collapse()"
+                >
                   {{#if child.isCollapsed}}
                     +
                   {{else}}