#15052: docs(ja-JP): add Japanese translations for install/ section

## Summary Adds Japanese translations for the core install documentation section. ### Files added - `docs/ja-JP/install/index.md` — Main install page - `docs/ja-JP/install/node.md` — Node.js setup guide - `docs/ja-JP/install/updating.md` — Update guide - `docs/ja-JP/install/uninstall.md` — Uninstall guide ### Translation approach - Mintlify component format preserved (AccordionGroup, Tabs, Note, Tip, Warning, etc.) - Code blocks and CLI commands kept in English - Descriptions, headings, and UI text translated to natural Japanese - Frontmatter `summary` and `title` translated; `read_when` kept in English for indexing ### Context Japanese documentation currently covers ~5% of English docs. This PR starts expanding coverage, beginning with the install section as the first touchpoint for new users. Related: #15049 🦞 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds four new Japanese (`ja-JP`) documentation pages covering the install section: an overview page plus Node.js setup, updating, and uninstall guides. The content preserves Mintlify components (Tabs/Accordions/Notes/etc.) and keeps command/code blocks in English while translating headings and descriptive text to Japanese, expanding the existing `docs/ja-JP/` docs coverage. <h3>Confidence Score: 4/5</h3> - Safe to merge after fixing a broken intra-page anchor link in the new install docs. - Changes are documentation-only and largely follow existing Mintlify patterns, but there is at least one definite dead link (`#install-methods`) that should be corrected to avoid confusing readers. - docs/ja-JP/install/index.md _{Last reviewed commit: 2914ed3} <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->