i18n(ja): add reference/routing-reference.mdx

[Yusaku01]

Description (required)

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

• Documents prerender, partial, and getStaticPaths() in Japanese.
• Includes params, props, routePattern, and pagination reference details.
• Updates internal links for the Japanese documentation.
• Updates one existing Japanese fragment link to match the translated heading.
• Follows the repository’s Japanese translation guide and existing documentation tone.

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