docs: update 2-13-e-invoicing-sdi.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟡 medium

What changed

Added a short section explaining how to send SDI invoices to Italian individual consumers (store the personal Codice Fiscale in the VAT number field and set the customer type to Consumer), plus a warning not to send live SDI invoices to Government/public-administration customers yet, since split-payment VAT handling isn't fully supported.

Source PRs

• light#7857 — Support B2C and B2G invoicing on the Italian SDI network

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.