docs/manual: move manual-prepare-sources rule into GENDOC

Copying the document's source files is a generic GENDOC action, that
will be common to all documents, so the dependency rule should be
handled in GENDOC, rather than letting all documents duplicate it.

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>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>