etcdctl: add page (#5006)

pages/common/etcdctl.md

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+# etcdctl
+
+> CLI interface for interacting with `etcd`, a highly-available key-value pair store.
+> Etcd stores data in hierarchically organized directories, as in a standard filesystem.
+> More information: <https://etcd.io/docs/latest/dev-guide/interacting_v3/>.
+
+- Display the value associated with a specified key:
+
+`etcdctl get {{my/key}}`
+
+- Store a key-value pair:
+
+`etcdctl put {{my/key}} {{my_value}}`
+
+- Delete a key-value pair:
+
+`etcdctl del {{my/key}}`
+
+- Store a key-value pair, reading the value from a file:
+
+`etcdctl put {{my/file}} < {{path/to/file.txt}}`
+
+- Save a snapshot of the etcd keystore:
+
+`etcdctl snapshot save {{path/to/snapshot.db}}`
+
+- Restore a snapshot of an etcd keystore (restart the etcd server afterwards):
+
+`etcdctl snapshot restore {{path/to/snapshot.db}}`
+
+- Add a user:
+
+`etcdctl user add {{my_user}}`
+
+- Watch a key for changes:
+
+`etcdctl watch {{my/key}}`