Docs: Remove config example from k8s install & link to config page

What does this MR do?

Following the discussion in !75366 (comment 744230291) this MR does the following:

• Remove the gitops config example, which is a Premium feature with manifests sync.
• Add URLs to manifest syns and network policies in the listing, to the configuration page. Over there the SSoT explains the configuration, and shows which features are in the Free and Premium/Ultimate tiers.
• Say that config.yaml can be empty to continue with the registration.
• Add a section to highlight the CI/CD tunnel

Related issues

!75366 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
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  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
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• Ensure a release milestone is set.