Mark 'key_count' as optional within info.json docs (#10513)

* Update reference_info_json.md

key_count is now an optional field because it is better to get the keycount by counting the elements in the layout array.

* Updated Japanese documentation as well

docs/ja/reference_info_json.md

@@ -36,7 +36,7 @@
 * `height`
    * オプション: キー単位でのレイアウトの高さ
 * `key_count`
-   * **必須**: このレイアウトのキーの数
+   * オプション: このレイアウトのキーの数
 * `layout`
    * 物理レイアウトを説明するキー辞書のリスト。詳細は次のセクションを見てください。
 

docs/reference_info_json.md

@@ -31,7 +31,7 @@ Within our `info.json` file the `layouts` portion of the dictionary contains sev
 * `height`
   * Optional: The height of the layout in Key Units
 * `key_count`
-  * **Required**: The number of keys in this layout
+  * Optional: The number of keys in this layout
 * `layout`
   * A list of Key Dictionaries describing the physical layout. See the next section for more details.