Document AppArmor mount permission error for rootless containers

What does this MR do and why?

Adds troubleshooting guidance for the failed to share mount point: /: permission denied error, which occurs when AppArmor blocks the mount syscall that rootlesskit-based rootless container tools (BuildKit, Podman, Docker) need on the Kubernetes executor.

This content previously lived only in the BuildKit CI docs, added in response to review feedback that Kubernetes-specific troubleshooting didn't belong in a doc/ci/docker page. Since the underlying issue comes from rootlesskit rather than BuildKit itself, it's moved here to the Kubernetes executor troubleshooting page, and the BuildKit docs now link to this section instead.

🔗 References

• Relates to Docs: buildkit docs do not work (gitlab#549941 - closed)
• Planning issue FY27Q2 Stage Lead planning issue: Verify (gitlab-org/technical-writing/team-tasks#1641 - closed)