README.md: use a dedicated zero-trust diagram, stop reusing the doc's

Was reusing netbird-zero-trust-mesh-topology.svg (the doc's detailed
peer-topology diagram) in the README too - breaks this repo's own
established convention of the README getting its own dedicated asset
even when covering related material (see readme-trust-boundaries-
blast-radius.svg vs architecture-overview-trust-depth.svg). New diagram
is a simple perimeter-vs-zero-trust conceptual contrast, matching the
README section's generalized framing rather than NetBird's specific
implementation topology.

assets/readme-zero-trust-model.svg

@@ -0,0 +1,4 @@
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