[enh] introduce /help route

Translation will be implemented in the future.
For now the "en" in /help/en/<pagename> is hardcoded.

tests/unit/test_webapp.py

@@ -174,7 +174,7 @@ class ViewsTestCase(SearxTestCase):
         self.assertIn(b'<description>first test content</description>', result.data)
 
     def test_about(self):
-        result = self.app.get('/about')
+        result = self.app.get('/help/en/about')
         self.assertEqual(result.status_code, 200)
         self.assertIn(b'<h1>About SearXNG</h1>', result.data)