Manifest: Create generate_all_doc for sub-libs

The following discussion from !9478 (merged) should be addressed:

• @raphael-proust started a discussion:

  Follow-up issue: make this more automatic. For exemple the Sub_lib.sub_lib function could take the doc header as parameter, it could save the package in a global ref, and then Sub_lib.generate_all_doc would iterate over that global ref.

  I'd suggest to open a follow-up issue.