Document for 3.6 release

[ahrtr]

We need to cover the following topics,

• Release blogs

  • Some features: v2store deprecation, downgrade, etc.
  • performance improvement (reduce memory usage):
    • Run a separate in memory snapshot to reduce number of entries stored in raft memory storage etcd#18825
    • Reuse events used for syncing watchers etcd#17563

• Upgrade checklist and guide

  • v2store deprecation: --enable-v2, --experimental-enable-v2v3 and other v2 proxy flags have already been removed in 3.6. Refer to [3.5] Print warning message for deprecated v2 flags if set etcd#18999
    • Users must remove these these flags before upgrading from 3.5 to 3.6. Also refer to Expected v2 deprecation behavior in HA clusters etcd#17009 (comment)
    • Run the tool to ensure there is no custom content in v2store: [3.5] Add online & offline commands to check whether the v2store contains custom content etcd#18993
  • downgrade stuff
  • anything else

• anything else

cc @jmhbnz @ivanvc