Docs: Refactor K8s Agent doc

What does this MR do?

Refactor the k8s Agent main doc. (1st iteration)

• Better intro. Include the "why", bring K8s, GitOps, GitLab, CD together.
• Move installation instructions into a new document.

Review apps:

• New installation page: http://main-ee-72149.178.62.207.141.nip.io/ee/user/clusters/agent/install/index.html
• Agent's main page: http://main-ee-72149.178.62.207.141.nip.io/ee/user/clusters/agent/index.html

In the next iterations, I plan to move the rest of the install-related content:

• Kubernetes Network Security Alerts into a new doc
• Remove the GitLab Kubernetes Agent => into a new doc
• Troubleshooting => into a new doc

Related issues

• &6267
• #329062 (closed)
• technical-writing#451 (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.

To-dos:

• Self-review
• Fix links
• PM review (or eng review)
• TW review
• FE review (if the changes affect help paths) => no help paths changed

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.