Document for 3.6 release

@jmhbnz

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 offline command to check whether the v2store contains custom content etcd#18993
- downgrade stuff
- anything else
The breaking change on the experimental flag as mentioned in Add server level feature gate etcd#18023 (comment)
anything else ?

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