docs: add systemd journal example to Promtail migration guide

[ferr079]

What this PR does

Adds a new example section to the Migrate from Promtail guide showing how to convert a systemd journal scrape configuration, which is one of the most common Promtail patterns on Linux hosts but isn't currently covered in the migration examples.

Why

The current guide only demonstrates converting static_configs + file-based scraping. For anyone running Promtail on bare Linux (VMs, LXC containers, physical servers), the primary use case is scraping systemd-journald — and nothing in the guide shows that mapping. New users have to piece the conversion together from loki.source.journal + loki.relabel reference pages without a concrete before/after example.

This PR adds a third example block (after the existing file-match example, before ## Limitations) with:

• A typical Promtail scrape_configs: journal: YAML
• The equivalent Alloy configuration using loki.source.journal and loki.relabel
• A note about the adm / systemd-journal group membership (already present on loki.source.journal.md but easy to miss during migration)

Context

Real-world source: I migrated ~50 systemd-based hosts from Promtail to Alloy in a homelab and went through this exact translation. The Alloy convert tool handles it but the doc gap makes the output harder to reason about for people doing the conversion by hand to understand what they're shipping.

Checklist

• Follows the existing "Example" section format in from-promtail.md
• Uses the same component references and admonition style as loki.source.journal.md
• Signed-off-by line included (DCO)

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