Add simplified architecture diagrams (#3557)

Problem: The current architecture documentation for NGF is written and presented in a way for a new user or developer to be able to understand.

Solution: Adding new diagrams and explanations for how traffic flows and how config changes are applied and the lifecycle of a gateway.

Author: sarthyparty

docs/architecture/README.md

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+# NGINX Gateway Fabric Architecture
+
+This guide explains how NGINX Gateway Fabric works in simple terms.
+
+## What is NGINX Gateway Fabric?
+
+NGINX Gateway Fabric (NGF) turns Kubernetes Gateway API resources into working traffic routing. It has two main parts:
+
+- **Control Plane**: Watches Kubernetes and creates NGINX configs
+- **Data Plane**: NGINX servers that handle your traffic
+
+## Control Plane vs Data Plane
+
+### Control Plane
+
+The **Control Plane** is the brain:
+
+- Watches for Gateway and Route changes in Kubernetes
+- Converts Gateway API configs into NGINX configs
+- Manages NGINX instances
+- Handles certificates and security
+
+### Data Plane
+
+The **Data Plane** does the work:
+
+- Receives incoming traffic from users
+- Routes traffic to your apps
+- Handles SSL/TLS termination
+- Applies load balancing and security rules
+
+## Architecture Diagrams
+
+### [Configuration Flow](./configuration-flow.md)
+
+How Gateway API resources become NGINX configurations
+
+### [Traffic Flow](./traffic-flow.md)
+
+How user requests travel through the system
+
+### [Gateway Lifecycle](./gateway-lifecycle.md)
+
+What happens when you create or update a Gateway
+
+For more detailed architectural information, see the [Gateway Architecture Overview](https://docs.nginx.com/nginx-gateway-fabric/overview/gateway-architecture/).
+
+## Key Concepts
+
+### Separation of Concerns
+
+- Control and data planes run in **separate pods**
+- They communicate over **secure gRPC**
+- Each can **scale independently**
+- **Different security permissions** for each
+
+### Gateway API Integration
+
+NGF implements Kubernetes Gateway API resources. For supported resources and their feature compatibility, see [Gateway API Compatibility](https://docs.nginx.com/nginx-gateway-fabric/overview/gateway-api-compatibility/).
+
+### NGINX Agent
+
+- **NGINX Agent v3** connects control and data planes
+- Runs inside each NGINX pod
+- Downloads configs from control plane
+- Manages NGINX lifecycle (start, reload, monitor)
+
+## Security Model
+
+### Control Plane Security
+
+- **Limited Kubernetes API access** (RBAC-controlled permissions to watch resources)
+- **gRPC server** for data plane connections
+- **Certificate management** for secure communication
+
+### Data Plane Security
+
+- **No Kubernetes API access** (security isolation)
+- **gRPC client** connects to control plane
+- **Minimal permissions** (principle of least privilege)

docs/architecture/configuration-flow.md

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+# Configuration Flow
+
+This diagram shows how Gateway API resources map to NGINX configurations.
+
+## Simple Overview
+
+```mermaid
+%%{init: {'theme':'dark', 'themeVariables': {'fontSize': '16px', 'darkMode': true, 'primaryColor': '#4f46e5', 'primaryTextColor': '#e5e7eb', 'primaryBorderColor': '#6b7280', 'lineColor': '#9ca3af', 'secondaryColor': '#1f2937', 'tertiaryColor': '#374151', 'background': '#111827', 'mainBkg': '#1f2937', 'secondBkg': '#374151', 'tertiaryTextColor': '#d1d5db'}}}%%
+graph TB
+    %% User Actions
+    USER[👤 User] --> K8S
+
+    %% Kubernetes API Layer
+    subgraph "Kubernetes API"
+        K8S[🔵 API Server]
+        GW[Gateway]
+        ROUTE[HTTPRoute]
+        SVC[Service]
+    end
+
+    %% Control Plane
+    subgraph "Control Plane Pod"
+        NGF[🎯 NGF Controller]
+    end
+
+    %% Data Plane
+    subgraph "Data Plane Pod"
+        AGENT[🔧 NGINX Agent]
+        NGINX[🌐 NGINX]
+        CONF[nginx.conf]
+    end
+
+    %% Flow
+    K8S --> NGF
+    GW --> NGF
+    ROUTE --> NGF
+    SVC --> NGF
+
+    NGF --> AGENT
+    AGENT --> CONF
+    CONF --> NGINX
+
+    %% Dark-friendly styling
+    style USER fill:#fbbf24,stroke:#f59e0b,stroke-width:2px,color:#1f2937
+    style NGF fill:#3b82f6,stroke:#2563eb,stroke-width:2px,color:#ffffff
+    style NGINX fill:#8b5cf6,stroke:#7c3aed,stroke-width:2px,color:#ffffff
+    style K8S fill:#6b7280,stroke:#4b5563,stroke-width:2px,color:#ffffff
+```
+
+## Step-by-Step Process
+
+### 1. User Creates Resources
+
+```yaml
+# User applies Gateway API resources
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  name: my-gateway
+```
+
+### 2. Kubernetes Stores Resources
+
+- Gateway, HTTPRoute, Service resources stored in etcd
+- Kubernetes API Server notifies controllers(watchers) of changes
+
+### 3. NGF Controller Processes Changes
+
+```text
+NGF Controller:
+├── Watches Gateway API resources
+├── Validates configurations
+├── Builds internal config graph
+└── Generates NGINX configuration
+```
+
+### 4. Configuration Sent to Data Plane
+
+```text
+Control Plane → Data Plane:
+├── gRPC connection (secure)
+├── nginx.conf file contents
+├── SSL certificates
+└── Other config files
+```
+
+### 5. NGINX Agent Updates Configuration
+
+```text
+NGINX Agent:
+├── Receives config from control plane
+├── Validates NGINX syntax
+├── Writes files to disk
+├── Tests configuration
+└── Reloads NGINX (if valid)
+```
+
+## Detailed Configuration Flow
+
+```mermaid
+%%{init: {'theme':'dark', 'themeVariables': {'fontSize': '14px', 'darkMode': true, 'primaryColor': '#4f46e5', 'primaryTextColor': '#e5e7eb', 'primaryBorderColor': '#6b7280', 'lineColor': '#9ca3af', 'secondaryColor': '#1f2937', 'tertiaryColor': '#374151', 'background': '#111827', 'actorBkg': '#374151', 'actorBorder': '#6b7280', 'actorTextColor': '#e5e7eb', 'activationBkgColor': '#4f46e5', 'activationBorderColor': '#3730a3', 'signalColor': '#9ca3af', 'signalTextColor': '#e5e7eb', 'labelBoxBkgColor': '#1f2937', 'labelBoxBorderColor': '#6b7280', 'labelTextColor': '#e5e7eb', 'loopTextColor': '#e5e7eb', 'noteBkgColor': '#374151', 'noteBorderColor': '#6b7280', 'noteTextColor': '#e5e7eb'}}}%%
+sequenceDiagram
+    participant User
+    participant API as K8s API
+    participant NGF as NGF Controller
+    participant Agent as NGINX Agent
+    participant NGINX
+
+    User->>API: Apply Gateway/Route
+    API->>NGF: Watch notification
+    NGF->>NGF: Validate resources
+    NGF->>NGF: Build config graph
+    NGF->>NGF: Generate nginx.conf
+    NGF->>Agent: Send config (gRPC)
+    Agent->>Agent: Write config files
+    Agent->>NGINX: Test config
+    Agent->>NGINX: Reload (if valid)
+    Agent->>NGF: Report status
+    NGF->>API: Update resource status
+```
+
+## What Gets Generated?
+
+### NGINX Configuration Files
+
+NGF generates various NGINX configuration files dynamically based on the Gateway API resources.
+
+### Example Generated Config
+
+```nginx
+# Generated from Gateway API resources
+server {
+    listen 80;
+    server_name api.example.com;
+
+    location /users {
+        proxy_pass http://user-service;
+    }
+
+    location /orders {
+        proxy_pass http://order-service;
+    }
+}
+```
+
+## Error Handling
+
+### Invalid Configuration
+
+1. **NGF validates** Gateway API resources
+2. **NGINX Agent tests** generated config
+3. **Rollback** if configuration is invalid
+4. **Status updates** report errors to Kubernetes
+
+### Recovery Process
+
+- Keep last known good configuration
+- Report errors in resource status
+- Retry configuration updates
+- Graceful degradation when possible