Static table of contents (#701)

* Add showing/hiding submenus, fix sidebar styling, fix bug where includes wouldn't appear in ToC

* Update ToC to highlight last header if page is scrolled to very bottom, fixes #280

* Set HTML title to current h1 section text, see #133

* Fix menu not opening on mobile

* Add back increase toc item height on mobile

* Fix padding bug

* Add back in ToC sliding animation

lib/toc_data.rb

@@ -0,0 +1,30 @@
+require 'nokogiri'
+
+def toc_data(page_content)
+  html_doc = Nokogiri::HTML::DocumentFragment.parse(page_content)
+
+  # get a flat list of headers
+  headers = []
+  html_doc.css('h1, h2, h3').each do |header|
+    headers.push({
+      id: header.attribute('id').to_s,
+      content: header.content,
+      level: header.name[1].to_i,
+      children: []
+    })
+  end
+
+  [3,2].each do |header_level|
+    header_to_nest = nil
+    headers = headers.reject do |header|
+      if header[:level] == header_level
+        header_to_nest[:children].push header if header_to_nest
+        true
+      else
+        header_to_nest = header if header[:level] == (header_level - 1)
+        false
+      end
+    end
+  end
+  headers
+end