docs: update 6-6-invoice-templates.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a "Good to know" callout documenting that typing {number} in the Default email subject/message auto-fills the invoice number, while the previously-suggested {{number}} double-brace form is not supported and stays literal. Did not document the AI-assistant email-template customization change (PR #7696), since there is no frontend evidence of how it appears in-app and this article currently doesn't reference AI-based template customization at all, so adding it risks inaccurate/unanchored guidance.

Source PRs

• light#7582 — fix/BLUE-1232/invoice-email-number-placeholder
• light#7696 — feat(BLUE-1331): invoice email template output with per-output tools and API (BLUE-1332)

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.