docs/manual: allow documents to define some hooks, as for a package

Allow documents to define two hooks:
  - DOC_CHECK_EXTRA_DEPENDENCIES_HOOKS
    to check for extra dependencies required by this document
  - DOC_CHECK_EXTRA_DEPENDENCIES_FMT_HOOKS
    ditto, but for the specific format
  - DOC_POST_EXTRACT_HOOKS:
    to run additional actions to fill-in the build directory

(Replace 'DOC' with the uppercase name of the document, and 'FMT' with
the format.)

This is supposed to replace the current use of overloading the internal
dependency rules, and makes GENDOC behave yet a bit more like the
package infrastructure.

Note that GENDOC_INNER already has the uppercase name as an argument,
whereas GENDOC does not, so the two foreach loops are not exactly
similar (for now.)

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 <patrickdepinguin@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>