Add feature gates doc for etcd

content/en/docs/v3.6/feature-gates/index.md

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+---
+title: Feature Gates
+weight: 10
+content_type: concept
+card:
+  name: reference
+  weight: 60
+---
+
+<!-- overview -->
+This page contains an overview of the various feature gates an administrator
+can specify on ETCD.
+
+See [feature stages](#feature-stages) for an explanation of the stages for a feature.
+
+
+<!-- body -->
+## Overview
+
+Feature gates are a set of key=value pairs that describe ETCD features.
+You can turn these features on or off using the `--feature-gates` command line flag
+on ETCD.
+
+ETCD lets you enable or disable a set of feature gates.
+Use `-h` flag to see a full set of feature gates.
+To set feature gates, use the `--feature-gates` flag assigned to a list of feature pairs:
+
+```shell
+--feature-gates=...,StopGRPCServiceOnDefrag=true
+```
+
+The following tables are a summary of the feature gates that you can set on
+ETCD.
+
+### Feature gates for Alpha or Beta features
+
+| Feature                          | Default | Stage |
+|----------------------------------|---------|-------|
+| StopGRPCServiceOnDefrag          | false   | Alpha |
+| InitialCorruptCheck              | false   | Alpha |
+| CompactHashCheck                 | false   | Alpha |
+| TxnModeWriteWithSharedBuffer     | true    | Beta  |
+
+## Using a feature
+
+### Feature stages
+
+A feature can be in *Alpha*, *Beta* or *GA* stage.
+An *Alpha* feature means:
+
+* Disabled by default.
+* Might be buggy. Enabling the feature may expose bugs.
+* Support for feature may be dropped at any time without notice.
+* The API may change in incompatible ways in a later software release without notice.
+* Recommended for use only in short-lived testing clusters, due to increased
+  risk of bugs and lack of long-term support.
+
+A *Beta* feature means:
+
+* Enabled by default.
+* The feature is well tested. Enabling the feature is considered safe.
+* Support for the overall feature will not be dropped, though details may change.
+* Recommended for only non-business-critical uses because of potential for
+  discovering new hard-to-spot bugs through wider adoption.
+
+**Note:**
+Please do try *Beta* features and give feedback on them!
+After they exit beta, it may not be practical for us to make more changes.
+
+A *General Availability* (GA) feature is also referred to as a *stable* feature. It means:
+
+* The feature is always enabled; you cannot disable it.
+* The corresponding feature gate is no longer needed.
+* Stable versions of features will appear in released software for many subsequent versions.
+
+A *Deprecated* feature means:
+
+* The feature gate is no longer in use.
+* The feature has graduated to GA or been removed.
+
+## List of feature gates {#feature-gates}
+
+Each feature gate is designed for enabling/disabling a specific feature.
+
+* `StopGRPCServiceOnDefrag`: Enable etcd gRPC service to stop serving client requests on defragmentation.
+* `TxnModeWriteWithSharedBuffer`: Enables the write transaction to use a shared buffer in its readonly check operations.
+* `InitialCorruptCheck`: Enable to check data corruption before serving any client/peer traffic.
+* `CompactHashCheck`: Enable leader to periodically check followers compaction hashes.