docs: explain the ten-year archiving obligation and where gofact fits

[btouchard]

Summary

• Adds docs/comprendre/archiver-dix-ans.md, the week 5 (comprendre la réforme) article: distinguishes the ten-year commercial archiving obligation (Code de commerce L123-22) from the six-year fiscal one (LPF L102 B), explains the authenticity/integrity/readability requirements, and situates gofact — local file, PDF/A-3, no archiving-service claim.
• Adds the article to mkdocs.yml navigation and to the llmstxt plugin sections, alongside the three existing "Comprendre la réforme" articles.

Test plan

• mkdocs build --strict passes locally, no warnings.
• Review by Benjamin before merge.