Allow fetching special files in the documentation (#1698)

In #1514, we added a symlink so that the docs would show up on the demo app …but
symlinks aren't supported on Windows and was causing a problem when trying to bundle.

This instead adds a lookup table of files not in docs/ which allows us to render those.

This partially fixes #1693, in allowing Administrate itself to be bundled without a symlink but
other dependencies have the same problem.

spec/example_app/app/controllers/docs_controller.rb

@@ -1,4 +1,11 @@
 class DocsController < ApplicationController
+  SPECIAL_FILES = [
+    {
+      file: 'CONTRIBUTING',
+      page: 'contributing'
+    }
+  ].freeze
+
   REDCARPET_CONFIG = {
     fenced_code_blocks: true,
     autolink: true,
@@ -9,11 +16,23 @@ def index
   end
 
   def show
-    render_page "docs/#{params[:page]}"
+    render_correct_page
   end
 
   private
 
+  def find_special_file
+    SPECIAL_FILES.select { |page| page[:page] == params[:page] }.first
+  end
+
+  def render_correct_page
+    if !find_special_file.nil?
+      render_page(find_special_file[:file])
+    else
+      render_page("docs/#{params[:page]}")
+    end
+  end
+
   def render_page(name)
     path = full_page_path(name)