#20198: docs: add Japanese localization for start section (AI-assisted)

This PR adds Japanese translations for the Getting Started section. ## Changes - Created `docs/ja-JP/start/` directory - Translated `getting-started.md`, `quickstart.md`, and `wizard.md` - `index.md` reviewed This contribution is AI-assisted. I have reviewed the translations for accuracy. <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR improves the Japanese (ja-JP) translations for the `docs/ja-JP/start/` section by updating `getting-started.md` and `wizard.md` to match the current English sources, and adding a new `quickstart.md` redirect page. The translation quality is good overall — content is faithful to the English source, links are valid, and Mintlify markup is correctly preserved. - **`x-i18n` metadata removed**: Both `getting-started.md` and `wizard.md` had their `x-i18n` frontmatter blocks stripped. This metadata contains the `source_hash` used by the `scripts/docs-i18n` pipeline to skip already-translated files. Without it, the next pipeline run will overwrite these manual translations with machine-generated ones. - **Glossary deviation**: "Skills" is translated to "スキル" throughout `wizard.md`, but the ja-JP glossary (`docs/.i18n/glossary.ja-JP.json`) specifies it should be kept as "Skills". - **New `quickstart.md`**: Clean redirect page matching the English source, no issues found. <h3>Confidence Score: 2/5</h3> - The missing x-i18n metadata will cause the i18n pipeline to overwrite these manual translations on the next run. - While the translations themselves are accurate, the removal of x-i18n frontmatter creates a pipeline compatibility issue that would silently undo this work. The glossary deviation is a secondary concern. - docs/ja-JP/start/getting-started.md and docs/ja-JP/start/wizard.md need their x-i18n metadata restored to prevent pipeline overwrites. _{Last reviewed commit: 2dfb809} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->