feat(modem_sim): Add initial modem simulator docs

common_components/modem_sim/README.md

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+# Modem Simulator Component
+
+A Wi-Fi modem simulator that extends ESP-AT with PPP server capabilities, turning ESP32 into a fully functional Wi-Fi modem. Perfect for testing AT commands and PPP connections without real hardware dependencies.
+
+## What it does
+
+- Extends ESP-AT firmware with PPP server functionality
+- Provides DATA mode for raw IP communication
+- Enables existing communication stacks (MQTT, HTTP, custom protocols) to work over Wi-Fi
+- Ideal for testing ESP-Modem library and CI reliability
+
+## Quick Start
+
+```bash
+cd common_components/modem_sim
+./install.sh
+source export.sh
+idf.py build
+```
+
+## Custom Platform/Module
+
+```bash
+./install.sh PLATFORM_ESP32S3 WROOM-32
+```
+
+## Configuration
+
+The `sdkconfig.defaults` includes:
+- Wi-Fi and Bluetooth enabled
+- PPP server support
+- AT commands for HTTP/MQTT
+- 4MB flash configuration
+
+## Project Structure
+
+```
+modem_sim/
+├── install.sh              # Installation script
+├── export.sh               # Environment setup
+├── sdkconfig.defaults      # Default configuration
+├── pppd_cmd/               # Custom PPP commands
+└── modem_sim_esp32/        # Generated ESP-AT build
+```
+
+## Use Cases
+
+- Testing ESP-Modem library without real hardware
+- Quick Wi-Fi connectivity for existing communication stacks
+- CI/CD testing with reliable modem simulation
+- Development and debugging of AT command implementations