docs/manual: rename the generic dependency rules

Those rules are generic, and not specific to our manual, we want them
always called:
  - manual-check-dependencies
  - manual-check-dependencies-pdf

So, rename them to be generic to gendoc.

Cascade the document's dependency checking to call those rules, too.

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