docs: Update build.sh for local builds
    
    There isn't an Antora docker container for aarch64. Given
    this is one of the supported fedora platforms it might be a
    good idea to provide a way to build docs on it.
    
    Antora may be installed locally, so lets check for a local
    copy before attempting to run a container to build the docs.
    
    Futher lets give aarch64 users a hint about this.
    
    Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>