Add docs for alerts from external Prometheus servers

What does this MR do?

Add docs for External Alerts, specify other docs are for managed instances only.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9836

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary

• Documentation created/updated via this MR

• Documentation reviewed by technical writer or follow-up review issue created

• Conforms to the style guides

• EE specific content should be in the top level /ee folder