docs: update 9-6-e-invoicing-compliance.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a note under "Party validation" in the Peppol validation rules section clarifying that German public-sector (Leitweg-ID) customers are exempt from the business registration number requirement that otherwise applies to German customers.

Source PRs

• light#7392 — fix(INTE-209): allow German government/Leitweg-ID customers to submit without a business registration number

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.