Add Capybara Page Status Code as a rails til.

rails/capybara-page-status-code.md

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+# Capybara Page Status Code
+
+To quickly determine if a page is rendering as expected or not, you can
+check the status code of the page. If your page is rendering successfully,
+you'll see something like this:
+
+```ruby
+> page.status_code
+# => 200
+```
+
+If some sort of application authorization logic is causing the page to not
+render as normal, you may see something like this:
+
+```ruby
+> page.status_code
+# => 403
+```