i18n(ja): add guides/routing.mdx

[Yusaku01]

Description (required)

Adds a Japanese translation of src/content/docs/en/guides/routing.mdx, based on the latest English documentation.

• Restores and updates the previous Japanese translation.
• Adds the Astro 7 advanced routing section.
• Updates routing, pagination, redirect, and code examples to match the current English page.
• Updates existing Japanese links to use the translated heading fragments.
• Follows the repository’s Japanese translation guide and existing documentation tone.

This draft targets this fork’s main branch for review before retargeting upstream.