Add congratulatory and condolence leave policy documentation

[Copilot]

This PR adds comprehensive documentation for congratulatory and condolence leave (慶弔休暇) to the company handbook as requested in the issue.

Changes Made

Added a new section "慶弔休暇" to /content/ja/company/day-off.md that includes:

Leave Entitlements:

• Marriage of employee: 5 days paid leave
• Wife's childbirth: 5 days paid leave
• Death of spouse, child, or parents: 5 days paid leave
• Death of siblings, grandparents, or spouse's family members: 3 days paid leave

Eligibility:

• Available only for permanent employees (無期雇用の従業員)
• All leave is paid (有給)

Application Process:

1. Employee: Request leave via Slack to supervisor
2. Back Office: Grant leave in Money Forward system
3. Employee: Apply for leave usage in Money Forward
4. Back Office: Review and approve the application

Important Notes:

• Only step 1 (initial request) needs to be done in advance
• Steps 2-4 can be completed after the fact for convenience

The documentation follows the existing handbook structure and formatting conventions, including proper use of Hugo shortcodes for alerts and consistent Japanese terminology.

Fixes #219.

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