doc/glossary.rst: add "Dashboard Plugin"

Add an entry below the (Mimic-era and therefore outdated but
nonetheless historically important) Dashboard Plugin key word in the
glosssary, which before now had never been added to the glossary.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/glossary.rst

@@ -261,6 +261,14 @@
                 Another name for :term:`Dashboard`.
 
 	Dashboard Plugin
+                The dashboard plugin was a Mimic-era web application that
+                visualized information and statistics about the Ceph cluster
+                using a web server hosted by the :ref:`Ceph
+                Manager<ceph-manager-daemon>`.
+
+                See `the Mimic-era Dashboard Plugin documentation
+                <https://docs.ceph.com/en/mimic/mgr/dashboard/>`_.
+
         DC
                 **D**\ata **C**\enter.