Automatically expand short in-page tables of contents (#367)

Most tables of contents within a single documentation page are quite
short, so having them start out collapsed just requires extra clicks.
This expands them just enough that they're no longer than
the (smallest possible version of) the global table of contents.

config.rb

@@ -81,6 +81,23 @@
       ul.elements.first.elements.first['href'] = a['href']
     end
 
+    # Expand the page's table of contents to the deepest level possible without
+    # making it longer than the most-collapsed-possible documentation table of
+    # contents.
+    entries = fragment.css("> li")
+    total_entries = entries.count
+    loop do
+      child_entries = entries.css("> ul > li")
+      total_entries += child_entries.count
+      break if total_entries > data.documentation.toc.count
+
+      sections = entries.xpath("a[following-sibling::ul]")
+      sections.add_class("section open")
+      break if sections.empty?
+
+      entries = child_entries
+    end
+
     current_page.add_metadata(table_of_contents: fragment.to_html)
     body
   end