[DOCS] Add docs for systemd logs

[Calm-Rock]

We currently don't have docs for sending SystemD logs to SigNoz.

Preliminary research:

• The SystemD logs are stored in JournalD
• OTel has a receiver for JournalD

Related Article - here

[Calm-Rock]

📝 Documentation Ready!

I've created comprehensive documentation for collecting SystemD logs (JournalD) in SigNoz. Here's what's included:

🔗 Pull Request: #1601

📚 What's Covered:

✅ Complete Setup Guide for both SigNoz Cloud and self-hosted deployments
✅ Prerequisites & Permissions setup for accessing journal logs
✅ OpenTelemetry JournalD Receiver configuration with detailed examples
✅ Advanced Filtering Options by SystemD units, priority levels, and custom matches
✅ Troubleshooting Section for common issues like permissions and performance
✅ Rich Metadata Documentation that SystemD logs provide

🛠 Technical Implementation:

• Uses OpenTelemetry JournalD receiver from opentelemetry-collector-contrib
• Follows the same structure as existing log collection guides
• Includes both YAML configuration examples and command-line instructions
• Documents rich metadata available in SystemD logs (_SYSTEMD_UNIT, PRIORITY, _PID, etc.)

📋 Key Sections:

1. Prerequisites - SystemD, JournalD, permissions setup
2. Installation - OpenTelemetry Collector Contrib setup
3. Configuration - JournalD receiver with filtering options
4. Deployment Scenarios - Same host, different host configurations
5. Advanced Configuration - Filtering by units, priority, custom matches
6. Troubleshooting - Permission issues, performance considerations
7. Rich Metadata - Understanding SystemD log fields

The documentation is ready for review and follows the same style and structure as other SigNoz log collection guides.

🔍 Preview:

The documentation will be available at /docs/userguide/collect_systemd_logs/ once merged.

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Ready for review! 🚀