OpenEBS - doc updates

[kmova]

• [x] update community/support page with CNCF slack
• [ ] review and update cstor user guides
• [x] alpha: add information about NDM feature gate
• [x] alpha: add zfs local pv
• [x] alpha : add Mayastor
• [x] alpha : update cStor v1 - CSI Driver
• [ ] alpha : include updates on Local PV monitoring

[kmova]

Local PV monitoring is still under evaluation and will be included later.

The alpha features sections have been updated: https://docs.openebs.io/docs/next/alphafeatures.html The support page has been updated: https://docs.openebs.io/docs/next/support.html

cStor guides update will be taken up in a different issue.