Update upgrading-fedora-offline.adoc, add instructions to regenerate rescue kernel (Issue #654)
    
    This commit provides instructions to regenerate the rescue kernel. The rescue kernel is generated by Anaconda during system install, but it may not be updated afterwards, like when updating a kernel or when performing a dnf-system-upgrade.

    

    It addresses discussions on the user mailing list, the Fedora boards and Reddit.[1,2.3]

    

    [1] https://discussion.fedoraproject.org/t/update-rescue-image

    [2] https://discussion.fedoraproject.org/t/questions-about-rescue-kernel

    [3] https://www.reddit.com/r/Fedora/comments/wfr5hu/about_the_rescue_mode_on_fedora/