docs: document vmauth port

Changed files
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docs
src
internal
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docs/src/internal/ports.md
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- `20xxx`: Prometheus/Metrics
- `2009x`: core metrics, node metrics
- `201xx`: service metrics
<!-- note: grafana uses unix sockets instead. -->
- `3xxxx`: Service ports
···
- `20xxx`: Prometheus/Metrics
- `2009x`: core metrics, node metrics
- `201xx`: service metrics
+
- `21000`: VMAuth (special case as this is not strictly metrics but a proxy)
<!-- note: grafana uses unix sockets instead. -->
- `3xxxx`: Service ports