docs: add Resource-Footprint page with real numbers from first VPS deploy
Document the disk/RAM/CPU footprint of a live deployment so anyone
sizing a VPS for DECNET can see what to expect. Numbers are from the
first Contabo deploy: 4.5 GiB disk, 2.1 GiB RAM, 0.03 load average,
12 containers, 12 workers, ~80 attackers in the first hour.
Adds a "what scales with topology size" breakdown so operators can
project from these numbers to their own target deployment, and a
sizing-floor recommendation per deployment shape (UNIHOST small,
UNIHOST medium, SWARM master, SWARM agent).
Linked from the User docs section of the sidebar between
Tailscale-Global-Deployment and MazeNET.
