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

[Yusaku01]

Description (required)

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

• Restores and updates the Japanese configuration reference.
• Covers current top-level, build, server, session, image, Markdown, i18n, environment, font, cache, and route rule options.
• Updates internal links for the Japanese documentation.
• Updates existing Japanese fragment links to match translated section headings.
• Follows the repository’s Japanese translation guide and existing documentation tone.

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