Fix the HTMLProofer build failures

This reverts commit 22558dd and adopts an alternative approach to the
problem. We continue to *NOT* run the link checker on the API docs (for
the reasons described in the commit messages for 22558dd), but we run
the HTMLProofer on the root of the output directory, so that internal
links can be properly resolved.

Rakefile

@@ -35,17 +35,13 @@ desc "Run the HTML-Proofer"
 task :run_proofer do
   require 'html-proofer'
 
-  paths_to_proof = Dir.glob('./output/*') - ['./output/api']
-  paths_to_proof.each do |path|
-    puts "Running HTML-Proofer on #{path}"
-
-    platform_hash_urls = ['#platform-mac', '#platform-windows', '#platform-linux', '#platform-all']
-    HTMLProofer.check_directory(path, {
-      :url_ignore => platform_hash_urls,
-      :typhoeus => { :ssl_verifypeer => false },
-      :verbose => true,
-    }).run
-  end
+  # Ignore platform switcher hash URLs
+  platform_hash_urls = ['#platform-mac', '#platform-windows', '#platform-linux', '#platform-all']
+  HTMLProofer.check_directory("./output", {
+    :file_ignore => [%r(/output/api/)],
+    :url_ignore => platform_hash_urls,
+    :typhoeus => { :ssl_verifypeer => false },
+  }).run
 end
 
 # Detects instances of Issue #204