doc: add section to ceph-installer(8)
doc: add section to ceph-installer(8)

Prior to this change, rst2man generated a man page header with a blank
section number:

  CEPH-INSTALLER()

Add the number so that rst2man will generate the correct header:

  CEPH-INSTALLER(8)

ceph/mariner-installer,ceph/ceph-installer,ceph/ceph-installer,ceph/ceph-installer