Rely on the plugin_manual mechanism in Foreman

Foreman 3.9 fixed this use and it simplifies the code. The plugin
already requires Foreman >= 3.10.

app/helpers/remote_execution_helper.rb

@@ -207,9 +207,8 @@ def time_in_words_span(time)
   end
 
   def documentation_button_rex(section = '')
-    url = 'http://theforeman.org/plugins/foreman_remote_execution/' +
-      "#{ForemanRemoteExecution::VERSION.split('.').take(2).join('.')}/index.html#"
-    documentation_button section, :root_url => url
+    version = ForemanRemoteExecution::VERSION.split('.').take(2).join('.')
+    documentation_button section, type: 'plugin_manual', name: 'foreman_remote_execution', version: version, section: "##{section}"
   end
 
   def description_checkbox_f(f, job_template, disabled)