doc/rados: add pg-states and pg-concepts to tree

Add "pg-states" and "pg-concepts" to the left tree pane on
docs.ceph.com.

This commit has been made in response to a request from the upstream
made in https://pad.ceph.com/p/Report_Documentation_Bugs.

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

Author: zdover23

doc/rados/operations/index.rst

@@ -39,6 +39,8 @@ CRUSH algorithm.
 	erasure-code
 	cache-tiering
 	placement-groups
+        pg-states
+        pg-concepts
 	upmap
         read-balancer
         balancer