Fix for link and note in secure clusters doc

What does this MR do?

Moves a note to fix it's placement in https://docs.gitlab.com/ee/user/project/clusters/securing.html#understanding-how-gitlab-managed-apps-are-installed

Also fixes an empty link missed during initial review https://docs.gitlab.com/ee/user/project/clusters/securing.html#install-gitlab-container-host-security

Related issues

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