process_toc.rb

require "jekyll-page-hooks"
#gem 'jekyll-page-hooks'

module Jekyll
  class ProcessTOC < PageHooks

    # Manipulate page/post data before it has been processed with Liquid or
    # Converters like Markdown or Textile.
    #
    def pre_render(page)
      #page.content = highlight_code(page.content)
    end

    # Manipulate page/post data after content has been processed to html.
    #
    def post_render(page)
      page.content = wrap_toc_in_details(page.content)
    end

    # Access page/post data after it has been successfully written to disk.
    #
    def post_write(page)
      #log_something(page.title)
    end

    # Plugin methods

    # Moves the generated <ul id="markdown-toc"> into a <details> element.
    def wrap_toc_in_details(content)
      content.gsub /\<\!--TOC_START--\>(.+)?\<\!--TOC_END--\>/m do
       <<-END
       <details id="toc">
         <summary>Table of contents</summary>
         #{$1}
       </details>
       END
      end
    end

  end
end