Guarded by a fortress of medieval walls cut from thick ominous sandstone, Les Archives Nationales means business. The former humble abode to a member of the French aristocracy, today this decadent sprawling chateau houses France’s most precious documents, each securely locked behind layers of mahogany-framed, etched-glass bookshelves. Formerly bequeathed the title “Hôtel de Soubise,” the…