doc/cephadm: break mon section into sections

doc/cephadm: break mon section into sections

This PR breaks the "Deploy Additional Monitors" section
of the cephadm documentation into several subsections
whose titles spotlight the matter under discussion in
those respective subsections.

inb4: Another PR is on deck that rewrites the sentences
in this chapter of the cephadm documentation. I'd like
to get this chapter broken up into these subsections before
I rewrite those sentences. So I'm hoping for no grammatical
mission creep on this one. The grammar and clarity updates
are coming.

Signed-off-by: Zac Dover <zac.dover@gmail.com>
(cherry picked from commit 25d9429d66a6edf446fd8bc3b7903b30de2aa31b)