docs: update references to renamed commons repository (#11)

services/docs/content/en/root/concepts/_index.md

@@ -0,0 +1,14 @@
+---
+title: Concepts
+description: Theoretical foundations, architecture, and security models.
+weight: 10
+---
+
+Theoretical foundations, architecture, and security models for the Root infrastructure framework.
+
+{{% cards %}}
+{{< card link="principles" title="Infrastructure principles" icon="book-open" subtitle="Predictability, minimal dependencies, and decentralization." >}}
+{{< card link="architecture" title="Infrastructure architecture" icon="share" subtitle="Decentralized orchestration and deployment models." >}}
+{{< card link="security-model" title="Security & privilege isolation" icon="shield-check" subtitle="Privilege isolation and network hardening." >}}
+{{< card link="tiered-storage" title="Tiered storage model" icon="database" subtitle="Standardized storage classification for service data." >}}
+{{% /cards %}}