[docs] Document the kuromoji (Japanese) inverted-index analyzer

[nishant94]

This PR adds documentation for the new built-in kuromoji parser for Japanese full-text search, mirroring how the existing chinese/ik analyzers are documented. Companion to the BE feature PR apache/doris@4.1.2.

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

New feature targeting the next release, so docs go to dev only, not backported into the released 4.x / 3.x / 2.1 versions.

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

[nishant94]

/review-docs