[webctl] Generate static data directory on upgrade (closes #2167873) - if the folder already exists, ``upgrade`` asks for deletion, - add an option (``generate-staticdir``) to allow skipping this task. - add an option (``staticdir-path``) to specify the static data folder path. The ``gen-static-datadir`` command allows to specify the target folder but there is otherwise no way to retrieve this information during upgrade.

../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc