update cstor documentation to explain how to avoid 2 ndm-operators

[survivant]

there is a little lack in the cstor-operator documentation in gh-pages/index.md

There should have a section that tell the user that the OpenEBS-operator Helm chart is not needed when we use cspc only.

if I do that :

helm install openebs -n openebs openebs/openebs --version 2.5.0 --set ndm.filters.enableOsDiskExcludeFilter=false --set ndm.sparse.count=5 --set apiserver.sparse.enabled=true --set featureGates.UseOSDisk.enabled=true
helm install -n openebs openebs-cstor openebs-cstor/cstor --version 2.5.0

I will obtain 2 npm-operators

NAME                        DESIRED   CURRENT   READY   UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE   LABELS
openebs-cstor-csi-node      4         4         4       4            4           <none>          17m   app.kubernetes.io/managed-by=Helm,chart=cstor-2.5.0,component=openebs-cstor-csi-node,heritage=Helm,name=openebs-cstor-csi-node,openebs.io/component-name=openebs-cstor-csi-node,openebs.io/version=2.5.0,release=openebs-cstor
openebs-cstor-openebs-ndm   4         4         4       4            4           <none>          17m   app.kubernetes.io/managed-by=Helm,app=openebs-ndm,chart=openebs-ndm-1.1.0,component=ndm,heritage=Helm,openebs.io/component-name=ndm,openebs.io/version=1.1.0,release=openebs-cstor
openebs-ndm                 4         4         4       4            4           <none>          22m   app.kubernetes.io/managed-by=Helm,app=openebs,chart=openebs-2.5.0,component=ndm,heritage=Helm,openebs.io/component-name=ndm,openebs.io/version=2.5.0,release=openebs
root@test-pcl109:~/setup-openebs/migration/2.5.0#

I could add a param to exclude npm with cstor-operator like this

helm install -n openebs openebs-cstor openebs-cstor/cstor --version 2.5.0 --set openebsNDM.enabled=false

but it's not clealy explain in the documentation.

and if we don't need openebs/openebs when using ctor-operator with cspc.. it should be written too.

and if we only use ctor-operator there should have a example to explain how to pass the value to ndm like this

helm install -n openebs openebs-cstor openebs-cstor/cstor --version 2.5.0 --set openebs-ndm.filters.enableOsDiskExcludeFilter=false