PVCs strategy in Che 7 docs are not up to date

[sleshchenko]

Is your task related to a problem? Please describe.

I've received a question from community about PVCs and checked Che docs related to it and figure it out that Che 7 actually has nothing
https://github.com/eclipse/che-docs/blob/7eb9099f445494fabd6cfbe13c307f96d2b1148f/src/main/pages/che-7/overview/con_persistent-volume-configuration.adoc

at the same time, Che 6 docs contain pretty good documentation about different PVCs strategies and how to configure them https://github.com/eclipse/che-docs/blob/2ca44011114cce9ef93963d633950289548371ce/src/main/pages/che-6/setup-kubernetes/kubernetes-or-openshift-admin-guide.adoc#storage-strategies-for-che-workspaces

So, I've created this issue to track and make sure that PVC strategy related information is migrated from Che 6 docs to Che 7 before Che 6 will be totally deprecated.

Maybe while Che 6 docs should be reviewed and migrated to Che 7, but it's another story and this issue is specific for PVCs strategies.

[sleshchenko]

cc @skabashnyuk @rkratky