Add system job template launch documentation

awx/api/templates/api/system_job_template_launch.md

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+Launch a Job Template:
+
+Make a POST request to this resource to launch the system job template.
+
+An extra parameter `extra_vars` is suggested in order to pass extra parameters
+to the system job task.  For example:   `{"days": 30}` to perform the action on
+items older than 30 days.
+
+If successful, the response status code will be 202.  If the job cannot be
+launched, a 405 status code will be returned.