docs: add Start From URL screenshots for start page documentation

Capture and add screenshots for the Start From URL modal:
- start_from_url_notebook.png (Import Notebook tab)
- start_from_url_github.png (Import GitHub Repository tab)
- start_from_url_gitlab.png (Import GitLab Repository tab)

Remove resolved TODO comments from all 4 language start docs.
Note: start_announcement_banner.png TODO remains as banner is not
configured in this environment.

Author: nowgnuesLee

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

@@ -48,7 +48,6 @@ external sources. Clicking the card opens a dialog with three tabs.
 ### Import Notebook
 
 ![](../images/start_from_url_notebook.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import Notebook tab -->
 
 1. Enter a Jupyter Notebook URL (must end with `.ipynb`) in the **Notebook URL**
    field
@@ -65,7 +64,6 @@ project documentation.
 ### Import GitHub Repository
 
 ![](../images/start_from_url_github.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import GitHub Repository tab -->
 
 1. Enter a valid GitHub repository URL in the **GitHub URL** field
 2. Select a **Storage Host** where the repository will be saved
@@ -78,7 +76,6 @@ when starting a session.
 ### Import GitLab Repository
 
 ![](../images/start_from_url_gitlab.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import GitLab Repository tab -->
 
 1. Enter a valid GitLab repository URL in the **GitLab URL** field
 2. Optionally specify a **GitLab Branch Name** (defaults to `master`)

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

@@ -47,7 +47,6 @@
 ### ノートブック取り込み
 
 ![](../images/start_from_url_notebook.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import Notebook tab -->
 
 1. **ノートブックのURL**フィールドにJupyter Notebook URL（`.ipynb`で終わる）を
    入力します
@@ -64,7 +63,6 @@ Markdownのバッジコードをコピーして、プロジェクトのドキュ
 ### GitHubリポジトリをインポートする
 
 ![](../images/start_from_url_github.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import GitHub Repository tab -->
 
 1. **GitHubのURL**フィールドに有効なGitHubリポジトリURLを入力します
 2. リポジトリを保存する**ストレージホスト**を選択します
@@ -78,7 +76,6 @@ Markdownのバッジコードをコピーして、プロジェクトのドキュ
 ### GitLabリポジトリ取り込み
 
 ![](../images/start_from_url_gitlab.png)
-<!-- TODO: Capture screenshot of the Start From URL modal showing the Import GitLab Repository tab -->
 
 1. **GitLabアドレス**フィールドに有効なGitLabリポジトリURLを入力します
 2. 必要に応じて**GitLab ブランチ名**を指定します（デフォルト: `master`）