docs: [#3] add comprehensive logging documentation for users and contributors

Co-authored-by: josecelano <[email protected]>

Author: Copilot

README.md

@@ -107,6 +107,44 @@ cargo install --path .
 torrust-tracker-deployer
 ```
 
+#### 📊 Logging Configuration
+
+The application includes comprehensive logging for observability and troubleshooting:
+
+```bash
+# Default behavior (production) - logs to file only in compact format
+torrust-tracker-deployer
+
+# Development mode - show logs on stderr in real-time
+torrust-tracker-deployer --log-output file-and-stderr
+
+# Use pretty format for readability during development
+torrust-tracker-deployer --log-format pretty --log-output file-and-stderr
+
+# JSON format for production monitoring and log aggregation
+torrust-tracker-deployer --log-format json
+
+# Custom log directory
+torrust-tracker-deployer --log-dir /var/log/deployer
+
+# Control log level with RUST_LOG environment variable
+RUST_LOG=debug torrust-tracker-deployer --log-output file-and-stderr
+RUST_LOG=trace torrust-tracker-deployer --log-output file-and-stderr
+```
+
+**Log Format Options:**
+
+- `compact` (default) - Space-efficient, readable format for production
+- `pretty` - Detailed, formatted output for development
+- `json` - Machine-readable format for log aggregation systems
+
+**Log Output Modes:**
+
+- `file-only` (default) - Write logs to `./data/logs/log.txt` only (production mode)
+- `file-and-stderr` - Write logs to both file and stderr (development/testing mode)
+
+**[📖 See detailed logging documentation →](docs/user-guide/logging.md)**
+
 #### 🔧 Development Tasks
 
 This project includes convenient scripts for common development tasks:

custom-logs/log.txt

@@ -0,0 +1,2 @@
+2025-10-15T12:42:14.572174Z  INFO torrust_tracker_deployer::app: Application started app="torrust-tracker-deployer" version="0.1.0" log_dir=./custom-logs log_format=Compact log_output=FileAndStderr
+2025-10-15T12:42:14.572262Z  INFO torrust_tracker_deployer::app: Application finished

docs/contributing/logging-guide.md

@@ -2,6 +2,63 @@
 
 This guide explains the structured logging implementation in the Torrust Tracker Deployer project, which uses hierarchical structured logging.
 
+## Main Application Logging
+
+The main CLI application (`src/main.rs` → `src/app.rs`) initializes logging at startup with user-configurable options. This provides a consistent logging infrastructure for all operations.
+
+### Application Logging Setup
+
+The main application uses `LoggingBuilder` with CLI arguments for configuration:
+
+```rust
+// src/app.rs
+use torrust_tracker_deployer_lib::logging::{LogFormat, LogOutput, LoggingBuilder};
+
+pub fn run() {
+    let cli = Cli::parse();
+
+    // Initialize logging FIRST before any other logic
+    LoggingBuilder::new(&cli.log_dir)
+        .with_format(cli.log_format)
+        .with_output(cli.log_output)
+        .init();
+
+    // Log startup with context
+    info!(
+        app = "torrust-tracker-deployer",
+        version = env!("CARGO_PKG_VERSION"),
+        log_dir = %cli.log_dir.display(),
+        log_format = ?cli.log_format,
+        log_output = ?cli.log_output,
+        "Application started"
+    );
+
+    // ... application logic ...
+
+    info!("Application finished");
+}
+```
+
+### User-Facing Configuration
+
+Users can configure logging via CLI arguments:
+
+```bash
+# Default (production): file-only, compact format
+torrust-tracker-deployer
+
+# Development: stderr output, pretty format
+torrust-tracker-deployer --log-format pretty --log-output file-and-stderr
+
+# Custom log directory
+torrust-tracker-deployer --log-dir /var/log/deployer
+
+# JSON format for log aggregation
+torrust-tracker-deployer --log-format json
+```
+
+See [User Guide: Logging](../user-guide/logging.md) for complete user documentation.
+
 ## JSON Output Format
 
 When using `logging::init_json()` or `LogFormat::Json`, logs are output in JSON format suitable for log aggregation: