docs: switch approved platforms without losing invoice history

[btouchard]

Summary

• New article docs/comprendre/changer-plateforme-historique.md: what the 27 July 2026 decree guarantees when switching approved platforms (PA) — portability of invoicing data, routing addresses, and invoice history — and what to check before switching (routing address update timing, in-flight invoices, testing flows before cutover).
• Adds the article to mkdocs.yml nav (both the Comprendre la réforme section and the llmstxt sections list).
• Deliberately omits the exact minimum-service duration owed by the departing platform: sources disagree (six vs. twelve months, moved during parliamentary debate) and the primary text could not be re-verified from this session (network egress to Légifrance is blocked here). The article tells readers to check the current duration directly rather than repeating an unverified figure.

Test plan

• mkdocs build --strict passes locally, no warnings.
• Reviewed against comms/GUARDRAILS.md in gofact-comms: no unverified regulatory figures, no claim that gofact is an approved platform, reference formula ("gofact produit le fichier ; votre plateforme agréée le transporte") present where transmission is discussed.