i18n(ja): add EmDash CMS guide

[Yusaku01]

Description (required)

Adds the missing Japanese translation of guides/cms/emdash.mdx, based on the current upstream English page (blob 602d0ddbb9b8a18d6de2e778ffda130bfde4c9df).

• Translates setup, dependencies, live collections, content rendering, and deployment.
• Preserves all code samples and admin UI labels.
• Uses Japanese internal links and the existing Japanese translation style.
• Changes only the Japanese EmDash page; no dependency changes.

Review

AI-assisted translation draft for native Japanese review in this fork only. Upstream's translation guidelines do not accept automated/machine translations; human review and compliance with those guidelines are required before any upstream submission.

Validation

• Target file passes Japanese textlint using the existing configuration.
• All fenced code samples match the English source exactly.
• No /en/ internal links remain.
• GitHub CI will be checked separately; Lunaria is excluded as requested.

Base: Yusaku01/docs:main (not upstream).