docs: update Japanese cell merging topics

[mkamiishi]

Summary

Updated the Japanese Cell Merging documentation to better align with the current English source.

Changes

• Updated the Angular Japanese Cell Merging topic.
• Updated the xplat Japanese Cell Merging shared topic.
• Removed duplicated English text that remained in the Japanese content.
• Added missing translated descriptions from the English source.
• Preserved code snippets, component names, property names, and enum values.

Testing

• Compared the Japanese Angular topic with the English source.
• Compared the Japanese xplat shared topic with the English source.
• Confirmed that code snippets, component names, property names, and enum values are preserved.

Checklist:

• check topic's TOC/menu and paragraph headings
• Include TOC topic labels in the topic content when it has a valuable update, is new, or is considered preview / beta
• link to other topics using ./page.mdx or ../relative/path.mdx (.mdx extension required)
• at the References section at the end of the topic add links to topics, samples, etc
• reference API documentation instead of adding a section with API

• use valid component names - [Data] Grid, IgxSelectComponent, <igx-combo>
• use spell checker tool (VS Code, Grammarly, Microsoft Editor)
• add inline code blocks for the names of classes / tags / properties
• add language descriptor for the code blocks
• check if links function by running the check-api/mdx npm commands
• check if sample is working and fully visible in the topic
• check if sample is working and fully visible in the StackBlitz
• check if code blocks match the code in StackBlitz demo
• add or update meaningful llms.description metadata and run npm run check:llms-metadata

• follow SEO guidelines to fill topic's metadata (SEO guidelines)

• do not resolve requested changes (leave that to the reviewer)

[mkamiishi]

Hi @ChronosSF,
I verified on staging, and it looks good. Please merge these changes on vnext to main and deploy to production whenever you get a chance. Thanks!