docs(kubernetes): document managed workspaces setup

[drew]

Summary

Add a dedicated Workspace on Kubernetes guide so operators can configure and verify managed per-workspace namespaces. The guide follows the current chart and driver behavior and links RFC 0011 for design background.

Related Issue

Closes #4621

Changes

• Explain shared, managed, and operator namespace mappings, including the current default.
• Document managed-mode Helm values, required RBAC, namespace lifecycle, verification, and private image-pull Secrets.
• Link the new guide from Kubernetes setup and the general Workspaces page.

Testing

• mise run docs passes (0 errors; 3 existing Fern warnings).
• Repository pre-commit checks pass.
• Unit tests added/updated (not applicable to documentation-only changes).
• E2E tests added/updated (not applicable to documentation-only changes).

Checklist

• Follows Conventional Commits.
• Commits are signed off (DCO).
• Architecture docs updated (not applicable; the page links RFC 0011).

[github-actions]

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