releng SOPs misc: drop dead pages, link sigul SOP, clean up template
    
    There's currently an `sop.adoc` page which looks like a bad wiki
    conversion of a list of links to the miscellaneous SOPs. We
    already have `index.adoc` as a *working* list of links to the
    miscellaneous SOPs, so this is pretty redundant, let's just drop
    it. Also dropped: sop_update_releng_docs , which is entirely
    outdated.
    
    Also, make `index.adoc` and the nav sidebar link to the sigul
    client setup SOP which they're currently missing, and update the
    SOP template page a bit.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>