Add Kubermatic Virtualization firewall rules how-to

[mihiragrawal]

What this PR documents:

Adds a "Firewall Rules" how-to for Kubermatic Virtualization, covering the dashboard
workflow for firewall rules: creating, editing and deleting rules from the Firewalls
page, the per-VM firewall switch on the Networking tab, port ranges, and the relaxed
enforcement mode that keeps ICMP (ping) and DHCP working on a firewalled VM. Also
documents the <vm>-default-deny baseline policy and the <vm>-allow-dns companion
policy that accompanies a target's first outbound rule.

This also introduces a tutorials-howtos section for Kubermatic Virtualization,
mirroring the existing KKP layout, and files the new page under it.

Which change does this document:

Documents the dashboard feature added in
kubermatic/kubermatic-virtualization#439 ("Add firewall rule create, edit and per-VM posture"),

How does this affect users:

Firewall rules were previously visible in the dashboard but read-only. Users can now
manage them from the UI. Rules are additive and allow-only, so blocking is expressed by
turning the firewall on and then permitting what you need; and the default relaxed
enforcement mode keeps ping and DHCP alive because Kubernetes NetworkPolicy cannot
express ICMP at all.

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