#773 French Header chapter needs significant updating

[Martin Holmes]

The French HD chapter was lacking the link to p5.nvdl, and therefore validity problems were not showing up. When I added the schema link, it revealed that the chapter is badly in need of work; it's still full of entity references instead of XIncludes. Since the structure of the English version may have changed a bit, someone will have to painstakingly work through the two and align the French one, pulling in the relevant bits.