i18n(ja): Add guides/sessions.mdx

[jp-knj]

Description (required)

This PR adds a complete Japanese (ja) translation of the Sessions guide.

• File added: src/content/docs/ja/guides/sessions.mdx
  (translated from src/content/docs/en/guides/sessions.mdx, see: fix: change access signature to Astro.session #11460 )
• Preserves all front-matter, Markdown structure, code blocks, and component imports.

This translation improves the documentation experience for Japanese-speaking developers and moves the Astro Docs project one step closer to full i18n coverage.

Related issues & labels (optional)

• Suggested label: i18n (or translation: ja)

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ja/guides/sessions.mdx	Localization added, will be marked as complete.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[jp-knj]

Unlike cookies, sessions are stored on the server, so you can keep larger amounts of data without worrying about size limits or security issues. They’re handy for persisting things like user data, shopping carts, and form state—all without any client-side JavaScript.

I’ve broken it up with periods for clarity.

[jp-knj]

“Interacting with session data” を直訳すると「セッションデータとのやり取り」や「セッションデータと対話する」といった不自然な見出しになる。ドキュメントの見出しとして読みやすく、内容も伝わる日本語にするなら、”セッションデータの扱い方”か”セッションデータを操作する”