docs: add announcement banner screenshot for start page

Capture start_announcement_banner.png using mocked announcement data
to demonstrate the banner feature. Remove resolved TODO comment from
start.md across all 4 languages.

Author: nowgnuesLee

packages/backend.ai-webui-docs/src/en/images/start_announcement_banner.png

@@ -15,7 +15,6 @@ formatting and may contain important notices about system maintenance, updates,
 or usage guidelines. You can dismiss the banner by clicking the close icon.
 
 ![](../images/start_announcement_banner.png)
-<!-- TODO: Capture screenshot of the announcement banner with sample content -->
 
 ## Action Cards
 

packages/backend.ai-webui-docs/src/en/start/start.md

@@ -15,7 +15,6 @@
 閉じるアイコンをクリックしてバナーを閉じることができます。
 
 ![](../images/start_announcement_banner.png)
-<!-- TODO: Capture screenshot of the announcement banner with sample content -->
 
 ## アクションカード
 

packages/backend.ai-webui-docs/src/ja/images/start_announcement_banner.png

@@ -13,7 +13,6 @@
 중요한 안내를 포함할 수 있습니다. 닫기 아이콘을 클릭하여 배너를 닫을 수 있습니다.
 
 ![](../images/start_announcement_banner.png)
-<!-- TODO: Capture screenshot of the announcement banner with sample content -->
 
 ## 액션 카드
 

packages/backend.ai-webui-docs/src/ja/start/start.md

@@ -15,7 +15,6 @@
 โดยคลิกไอคอนปิด
 
 ![](../images/start_announcement_banner.png)
-<!-- TODO: Capture screenshot of the announcement banner with sample content -->
 
 ## การ์ดการดำเนินการ