doc: use ref for links in add-or-rm-mons.rst

Add a label in doc/dev/mon-bootstrap.rst and use ref for all internal
links in doc/rados/operations/add-or-rm-mons.rst.
Change very complex labels in add-or-rm-mons.rst to simpler ones and fix
some inconsistent indentation.

Remove a stray ??? string at the end of a paragraph.
Use sudo when running systemctl.

Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>