docs: Add signature query parameter to docs of relevant endpoints (#1955)

### Description:
Update docs with signature query parameter for the following endpoints:
- /key-value-store-record-get
- /key-value-store-keys-get
- /dataset-items-get


### Issues:

Closed: #1954

Author: Pijukatel

apify-api/openapi/paths/datasets/datasets@{datasetId}@items.yaml

@@ -367,6 +367,14 @@ get:
       schema:
         type: boolean
         example: false
+    - name: signature
+      in: query
+      description: Signature used to access the items.
+      required: false
+      style: simple
+      schema:
+        type: string
+        example: 2wTI46Bg8qWQrV7tavlPI
   responses:
     '200':
       description: ''

apify-api/openapi/paths/key-value-stores/key-value-stores@{storeId}@keys.yaml

@@ -48,6 +48,14 @@ get:
       schema:
         type: string
         example: post-images-
+    - name: signature
+      in: query
+      description: Signature used to access the keys.
+      required: false
+      style: simple
+      schema:
+        type: string
+        example: 2wTI46Bg8qWQrV7tavlPI
   responses:
     '200':
       description: ''

apify-api/openapi/paths/key-value-stores/key-value-stores@{storeId}@records@{recordKey}.yaml

@@ -32,6 +32,14 @@ get:
       schema:
         type: string
         example: someKey
+    - name: signature
+      in: query
+      description: Signature used to access the record.
+      required: false
+      style: simple
+      schema:
+        type: string
+        example: 2wTI46Bg8qWQrV7tavlPI
   responses:
     '200':
       description: ''