Docs: Move clusters index content into infra/clusters

What does this MR do?

• Move the content of user/project/clusters/index.md into user/infrastructure/clusters/index.md.
• Re-write content of user/project/clusters/index.md to a project-level only overview (MVC).

Review apps:

• New cluster's landing page: http://main-ee-71588.178.62.207.141.nip.io/ee/user/infrastructure/clusters/index.html
• Content left on the project-level clusters page: http://main-ee-71588.178.62.207.141.nip.io/ee/user/project/clusters/index.html

Next iterations:

• Create a new "manage" page under infrastructure/clusters/manage/index.md
• Create a new "deploy" page under infrastructure/clusters/deploy/index.md
• Possibly move the content of "cluster levels" into the new index page (infrastructure/clusters/index.md)
• Possibly move the content of "supported K8s clusters" into the new index page (infrastructure/clusters/index.md)

Related issues

• For &6499 (closed)
• Closes #339852 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • 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.

My checklist:

• Move content to new page
• Update content "old" page
• Self-review
• TW review
• Fix links and anchors
• PM review
• Prep nav update (gitlab-docs!2181 (merged))

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• 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.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • 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.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.