middleman instance variables that are collections of pages

config.rb

# ...

def get_pages
  # get an array of pages whose source filename contains .html
  @pages = sitemap.resources.find_all { |page| page.source_file.match(/\.html/) }

  # sort them by an arbitrary YAML frontmatter property called 'order'
  @pages.sort! { |a,b| a.data['order'].to_i <=> b.data['order'].to_i }
end

ready do
  get_pages
end

# ...