docs: [#238] refactor user guide to extract Prometheus content into service-specific guide

- Created docs/user-guide/services/ directory for service documentation
- Moved Prometheus configuration and usage details to services/prometheus.md
- Added services/README.md explaining the directory structure and purpose
- Updated main user guide to remove Prometheus-specific content
- Added Services section to user guide with links to service guides
- Simplified configuration examples to show only core fields

This improves maintainability by:
- Keeping generic deployment workflow in main guide
- Isolating service-specific details in dedicated guides
- Making it easier to add/remove services from the stack
- Following DRY principle established in E2E testing docs

Author: josecelano

docs/user-guide/README.md

@@ -10,6 +10,7 @@ Welcome to the Torrust Tracker Deployer user guide! This guide will help you get
 - [Available Commands](#available-commands)
 - [Basic Workflows](#basic-workflows)
 - [Configuration](#configuration)
+- [Services](#services)
 - [Troubleshooting](#troubleshooting)
 - [Additional Resources](#additional-resources)
 
@@ -236,9 +237,6 @@ The environment configuration file is in JSON format:
     "public_key_path": "/path/to/public/key",
     "username": "ssh-username",
     "port": 22
-  },
-  "prometheus": {
-    "scrape_interval": 15
   }
 }
 ```
@@ -274,55 +272,46 @@ The environment configuration file is in JSON format:
 - SSH port number
 - Default: `22`
 
-**prometheus.scrape_interval** (optional):
+For service-specific configuration (Prometheus, MySQL, etc.), see the [Services](#services) section below.
 
-- Metrics collection interval in seconds
-- Default: `15` (included in generated templates)
-- Prometheus service enabled by default for monitoring
-- To disable: Remove the entire `prometheus` section from config
+## Services
 
-### Monitoring with Prometheus
+The Torrust Tracker Deployer supports optional services that can be enabled in your deployment:
 
-The deployer includes Prometheus for metrics collection by default. Prometheus automatically scrapes metrics from the tracker's HTTP API endpoints.
+### Available Services
 
-**Default Behavior**:
+- **[Prometheus Monitoring](services/prometheus.md)** - Metrics collection and monitoring (enabled by default)
+  - Automatic metrics scraping from tracker API
+  - Web UI on port 9090
+  - Configurable scrape intervals
+  - Can be disabled by removing from configuration
 
-- Prometheus is **enabled by default** in generated environment templates
-- Metrics collected from both `/api/v1/stats` and `/api/v1/metrics` endpoints
-- Accessible via web UI on port `9090`
+### Adding or Removing Services
 
-**Configuration**:
+Services are configured in your environment JSON file. To enable a service, include its configuration section. To disable it, remove the section.
+
+**Example with Prometheus**:
 
 ```json
 {
+  "environment": { "name": "my-env" },
+  "ssh_credentials": { ... },
   "prometheus": {
     "scrape_interval": 15
   }
 }
 ```
 
-**Disabling Prometheus**:
-
-To deploy without Prometheus monitoring, remove the entire `prometheus` section from your environment config:
+**Example without Prometheus**:
 
 ```json
 {
   "environment": { "name": "my-env" },
   "ssh_credentials": { ... }
-  // No prometheus section = monitoring disabled
 }
 ```
 
-**Accessing Prometheus**:
-
-After deployment, access the Prometheus UI at `http://<vm-ip>:9090` where you can:
-
-- View current metrics from tracker endpoints
-- Query historical data
-- Check target health status
-- Explore available metrics
-
-See [Prometheus Verification Guide](../e2e-testing/manual/prometheus-verification.md) for detailed verification steps.
+See individual service guides for detailed configuration options and verification steps.
 
 ### Logging Configuration
 

docs/user-guide/services/README.md

@@ -0,0 +1,111 @@
+# Services Documentation
+
+This directory contains detailed documentation for optional services that can be included in your Torrust Tracker deployments.
+
+## Purpose
+
+The services documentation provides comprehensive guides for each optional service, including:
+
+- Configuration options and examples
+- Enabling/disabling instructions
+- Verification and testing procedures
+- Troubleshooting common issues
+- Architecture and deployment details
+
+## Available Services
+
+- **[Prometheus Monitoring](prometheus.md)** - Metrics collection and monitoring service
+  - Automatic metrics scraping from tracker API endpoints
+  - Web UI for querying and visualizing metrics
+  - Configurable scrape intervals
+  - Enabled by default, can be disabled
+
+## Service Organization
+
+Each service guide follows a consistent structure:
+
+1. **Overview** - Purpose and capabilities
+2. **Default Behavior** - Out-of-the-box configuration
+3. **Configuration** - How to configure the service
+4. **Disabling** - How to remove the service from deployment
+5. **Accessing** - How to interact with the service after deployment
+6. **Verification** - How to verify the service is working correctly
+7. **Troubleshooting** - Common issues and solutions
+8. **Architecture** - Technical details about deployment structure
+
+## How Services Work
+
+Services in the deployer are:
+
+- **Optional** - Include only what you need
+- **Configuration-based** - Enable by adding a section to your environment JSON
+- **Containerized** - Each service runs in its own Docker container
+- **Integrated** - Automatically configured to work with the tracker
+
+### Adding a Service
+
+To include a service in your deployment, add its configuration section to your environment JSON file:
+
+```json
+{
+  "environment": {
+    "name": "my-env"
+  },
+  "ssh_credentials": {
+    "private_key_path": "~/.ssh/id_rsa",
+    "public_key_path": "~/.ssh/id_rsa.pub",
+    "username": "torrust"
+  },
+  "prometheus": {
+    "scrape_interval": 15
+  }
+}
+```
+
+### Removing a Service
+
+To exclude a service from your deployment, simply remove its configuration section:
+
+```json
+{
+  "environment": {
+    "name": "my-env"
+  },
+  "ssh_credentials": {
+    "private_key_path": "~/.ssh/id_rsa",
+    "public_key_path": "~/.ssh/id_rsa.pub",
+    "username": "torrust"
+  }
+  // No prometheus section = service not deployed
+}
+```
+
+## Future Services
+
+As the deployer evolves, additional optional services may be added to this directory:
+
+- Database services (MySQL, PostgreSQL)
+- Reverse proxy services (Nginx, Traefik)
+- Logging aggregation (Loki, Elasticsearch)
+- Alerting services (Alertmanager)
+- Visualization services (Grafana)
+
+## Related Documentation
+
+- **[User Guide](../README.md)** - Main user guide with general configuration
+- **[Quick Start Guide](../quick-start.md)** - Getting started with deployments
+- **[Configuration Reference](../configuration/)** - Environment configuration details
+- **[Manual Testing Guides](../../e2e-testing/manual/)** - Service verification procedures
+
+## Contributing
+
+When adding new service documentation:
+
+1. Follow the established structure outlined above
+2. Include practical examples and commands
+3. Provide verification steps
+4. Document common troubleshooting scenarios
+5. Update this README to list the new service
+6. Add cross-references to related documentation
+
+See [Contributing Guidelines](../../contributing/README.md) for more details.