docs/asciidoc: make it possible to use $(@D) in hooks

Currently, it is not possible to use $(@D) in documents' hooks, because
there is no actual target file for the copying rule.

So, use the same mechanism as for generic-package.

We do not touch the target file, so it is easy to regenerate the manual
without calling the -clean rule first.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>