Update CI/CD variables help text

Add hint about adding variables for Auto DevOps application variables
and link to More information

app/views/ci/variables/_content.html.haml

 = _('Environment variables are applied to environments via the runner. They can be protected by only exposing them to protected branches or tags. You can use environment variables for passwords, secret keys, or whatever you want.')
+= _('You may also add variables that are made available to the running application by prepending the variable key with <code>K8S_SECRET_</code>.').html_safe
+= link_to _('More information'), help_page_path('ci/variables/README', anchor: 'variables')

locale/gitlab.pot

@@ -7719,6 +7719,9 @@ msgstr ""
 msgid "You have reached your project limit"
 msgstr ""
 
+msgid "You may also add variables that are made available to the running application by prepending the variable key with <code>K8S_SECRET_</code>."
+msgstr ""
+
 msgid "You must accept our Terms of Service and privacy policy in order to register an account"
 msgstr ""