Update images for the 'Project users' documentation

doc/workflow/add-user/add-user.md

doc/workflow/add-user/img/add_user_email_accept.png

doc/workflow/add-user/img/add_user_email_ready.png

doc/workflow/add-user/img/add_user_email_search.png

doc/workflow/add-user/img/add_user_give_permissions.png

...user/img/add_user_import_members_from_another_project.png

doc/workflow/add-user/img/add_user_imported_members.png

doc/workflow/add-user/img/add_user_list_members.png

doc/workflow/add-user/img/add_user_members_menu.png

doc/workflow/add-user/img/add_user_search_people.png