doc/glossary: add "Quorum" to glossary

doc/glossary: add "Quorum" to glossary

Add the term "Quorum" to the glossary and link to the part of
architecture.rst concerning Monitors. The sticky header at the top of
the docs.ceph.com website gets in the way of the location linked to in
this commit, but fatigue and disgust prevent me from spending time today
trial-and-erroring my way through the hostile and ill-documented
wilderness of scroll-margin so that the link goes where it should.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit c2f6a770bf0e12296c334d99ac86ff4732ec29b7)