docs: update monitoring index

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

docs/index.rst

@@ -7,7 +7,7 @@ This is official etcd documentation.
 Still working in progress...
 
 * :ref:`set-up`: setting up an etcd cluster.
-* :ref:`monitor`: monitoring an etcd cluster.
+* :ref:`operate`: operating an etcd cluster.
 * :ref:`client-architecture`: describes etcd client components.
 
 .. toctree::
@@ -22,12 +22,6 @@ Still working in progress...
 
    operate
 
-.. toctree::
-   :maxdepth: 3
-   :caption: Monitor
-
-   monitor
-
 .. toctree::
    :maxdepth: 3
    :caption: FAQs