add API docs for KVv2 subkeys endpoint (#13893)

* add API docs for KVv2 subkeys endpoint

* add changelog entry

changelog/13893.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+secrets/kv: add subkeys endpoint to retrieve a secret's stucture without its values
+```

website/content/api-docs/secret/kv/kv-v2.mdx

@@ -274,6 +274,69 @@ $ curl \
 }
 ```
 
+## Read Secret Subkeys
+
+This endpoint provides the subkeys within a secret entry that exists
+at the requested path. The secret entry at this path will be retrieved
+and stripped of all data by replacing underlying values of leaf keys
+(i.e. non-map keys or map keys with no underlying subkeys) with `null`.
+
+| Method | Path                    |
+| :----- | :---------------------- |
+| `GET`  | `/secret/subkeys/:path` |
+
+### Parameters
+
+- `path` `(string: <required>)` – Specifies the path of the secret to read.
+  This is specified as part of the URL.
+- `version` `(int: 0)` - Specifies the version to return. If not set the latest
+  version is returned.
+- `depth` `(int: 0)` - Specifies the deepest nesting level to provide in the output.
+  The default value 0 will not impose any limit. If non-zero, keys that reside at the
+  specified `depth` value will be artificially treated as leaves and will thus be `null`
+  even if further underlying subkeys exist.
+
+### Sample Request
+
+```shell-session
+$ curl \
+    --header "X-Vault-Token: ..." \
+    https://127.0.0.1:8200/v1/secret/subkeys/my-secret?version=1
+```
+
+### Sample Secret Data
+
+```json
+{
+  "foo": "abc",
+  "bar": {
+    "baz": "def"
+  },
+  "quux": {}
+}
+```
+
+### Sample Response
+
+```json
+{
+  "subkeys": {
+    "foo": null,
+    "bar": {
+      "baz": null
+    },
+    "quux": null
+  },
+  "metadata": {
+    "created_time": "2021-12-14T20:28:00.773477Z",
+    "custom_metadata": null,
+    "deletion_time": "",
+    "destroyed": false,
+    "version": 1
+  }
+}
+```
+
 ## Delete Latest Version of Secret
 
 This endpoint issues a soft delete of the secret's latest version at the
@@ -562,6 +625,7 @@ $ curl \
 ```
 
 ## Patch Metadata
+
 This endpoint patches an existing metadata entry of a secret at the specified
 location. The calling token must have an ACL policy granting the `patch`
 capability. Currently, only JSON merge patch is supported and must be specified
@@ -599,7 +663,7 @@ not create a new version.
 {
   "max_versions": 5,
   "custom_metadata": {
-    "bar": "123",
+    "bar": "123"
   }
 }
 ```