docs: explain what a Factur-X file actually contains

[btouchard]

What

Adds this week's deep-dive article to docs/comprendre/: Anatomie d'un Factur-X : ce qu'il y a vraiment dans le fichier.

Content

• The two layers of a Factur-X file: the PDF/A-3 wrapper and the embedded CII XML.
• Where Factur-X sits relative to UBL and plain CII.
• The EN 16931 field nomenclature, with BT-34/BT-49 (routing addresses) called out specifically.
• Why reconstructing the XML through an internal model — instead of embedding it verbatim — is the most common way those routing addresses get silently dropped.
• The distinction between a valid file and an accepted one.
• Sources cited (Légifrance decree/arrêté of 27 July 2026, data.gouv.fr PA dataset, EN 16931 explainer).

Checks

• No platform-agreed-count figures cited, per the guardrail on unstable numbers.
• No claim that gofact transmits to the administration; the "gofact produces the file, your approved platform carries it" line is present.
• Wired into both mkdocs.yml nav entries under Comprendre la réforme.
• ~1100 words, within the 900–1600 target.