Fix outdated Japanese documents

[takashiuesaka]

fixed #465

[Copilot]

Pull request overview

This PR updates Japanese documentation to align with current Aspire content, including the Aspire 13 “What’s new” page and testing guidance, and refreshes the Japanese landing pages to promote Aspire Conf.

Changes:

• Updates /ja/whats-new/aspire-13 content and formatting, including adding the DefaultAzureCredential breaking-change section.
• Updates Japanese testing docs to reflect newer APIs for environment variable evaluation and improves Japanese-only text consistency.
• Refreshes the Japanese home banner and the /ja/aspireconf page with updated event content and embedded livestream tabs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/frontend/src/content/docs/ja/whats-new/aspire-13.mdx	Updates Japanese Aspire 13 release notes, adds new section, and adjusts links/formatting.
src/frontend/src/content/docs/ja/testing/write-your-first-test.mdx	Updates test examples to use ExecutionConfigurationBuilder for env var evaluation and cleans up mixed-language bullets.
src/frontend/src/content/docs/ja/testing/manage-app-host.mdx	Adjusts headings and Japanese text around AppHost management in tests.
src/frontend/src/content/docs/ja/index.mdx	Updates the Japanese homepage banner to promote Aspire Conf.
src/frontend/src/content/docs/ja/aspireconf/index.mdx	Expands the Japanese Aspire Conf page with sessions grid and YouTube/Twitch embed tabs.

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