Add observability architecture documentation

This document outlines the architecture of Ced's Observability Stack, detailing its components and their interactions.

architecture/observability-architecture.md

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+```markdown
+# Observability Architecture
+
+## Purpose
+
+This document explains the architecture of Ced’s Observability Stack.
+
+The observability stack collects, stores, and visualizes metrics from Ced’s HomeLab infrastructure.
+
+---
+
+## High-Level Design
+
+```mermaid
+flowchart LR
+    subgraph Infrastructure
+        PVE[Proxmox HA Cluster]
+        K3S[12-Node K3s Cluster]
+        TN[TrueNAS]
+        NPM[Nginx Proxy Manager]
+        SVC[Homelab Services]
+    end
+
+    subgraph Metrics
+        NE[Node Exporter]
+        PME[Proxmox Exporter]
+        KSM[Kube-State-Metrics]
+        HC[Health Check Scripts]
+    end
+
+    subgraph Observability
+        PROM[Prometheus]
+        GRAF[Grafana]
+        ALERT[Alertmanager]
+    end
+
+    PVE --> PME
+    K3S --> KSM
+    SVC --> HC
+    TN --> HC
+    NPM --> HC
+
+    PME --> PROM
+    KSM --> PROM
+    NE --> PROM
+    HC --> PROM
+
+    PROM --> GRAF
+    PROM --> ALERT
+
+    GRAF --> NOC[Ced's NOC]