Agent for Kubernetes docs update for manifest sync in Premium

What does this MR do?

https://forum.gitlab.com/t/gitlab-ce-14-5-0-kubernetes-agent/61844/12 says that the docs in https://docs.gitlab.com/ee/user/clusters/agent/install/ imply that everything is in Free.

I started analysing the forum topic, to understand the user's flow how they came to a non-working state.

We provide a minimal example configuration, which uses the gitops keyword and shows how to sync manifests. From there, it is not clear that this is a Premium feature; you really need to click on the URL to see the Premium tier chapter marker over there.

Following the path, the user copied the gitops configuration for their CE free tier setup. This errored out on the KAS server, and led to debugging questions.

This MR is to address the example configuration, and split it into two parts:

• Show the CI/CD tunnel access (free tier)
• Show the manifest sync gitops config (Premium marker)

That way, users can decide the flow in the documentation whether to use the free or premium configuration, and learn more in the linked chapter then.

The MR introduces a bit of config duplication, but avoids confusion until we decide whether to move the gitops feature to all tiers in #346567 (closed)

Related issues

#346583 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
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If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
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These labels cause the MR to be added to code verification QA issues.

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