InfluxDB 3 systemd integration

# Kapa.ai Coverage Gap: InfluxDB 3 systemd integration

## Finding
All Linux users asked for a systemd unit to control InfluxDB 3 with systemctl. The bot only repeated that no official file exists and provided no concrete template, installation flags, or roadmap, so users still lack a workable, supported way to start the server automatically at boot.

## Suggestion
Update docs to state the current support status, supply a vetted sample unit file (or explain why it’s unsupported), list required environment variables and restart policies, and outline any future plans for native systemd support.

## Conversations
- give me the systemd service file to manage influxdb3 core
- how to configure influxDB 3 as systemctl
- one example of systemd service for influxdb 3?

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Explore user conversations on:
https://app.kapa.ai/720100f9-ce93-4305-88fd-5fcf71effad7/coverage-gaps/clusters/bf1b7803-a4d9-4e8a-ae79-ddb7660700c5

Start by reviewing the existing InfluxDB 3 documentation and the linked Kapa.ai conversations to identify the current systemd support gap. Done means the docs clearly state support status, provide a vetted unit example or explain its absence, cover required environment variables and restart policies, and mention future plans.

Written by the indexing model from the issue text.