Resolve "Add a mention and link to doc for security implications in the Kubernetes cluster page"

What does this MR do?

Adds a paragraph about security concerns on the Cluster show page

Why was this MR needed?

Runners are going to be enabled with privilege mode by default, we want to make users aware of possible security implications.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by UX
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43780 (closed), https://gitlab.com/gitlab-org/gitlab-ce/issues/43781

Note

The link Learn more about security configuration has an anchor for #security-implications but it's currently redirecting to the clusters documentation page, since security implications section is being currently address on https://gitlab.com/gitlab-org/gitlab-ce/issues/43781