i18n(ja): fix Region/Store/Leader translations in TiKV context

[yahonda]

What is changed, added or deleted? (Required)

Fix Japanese translations in TiKV technical context across 38 files:

• 地域 → リージョン (TiKV Region / Raft group): 14 files, 18 occurrences
• 店舗 → ストア (TiKV Store): 29 files, 63 occurrences
• Leader → リーダー (Raft leader): 2 files, 6 occurrences

Also fixed a glossary entry (店 → ストア) and improved a phrase (一般的なビュー → 概要) in explain-walkthrough.md.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request standardizes the Japanese translation of "Region" to "リージョン" (instead of "地域") across various documentation files. The review comments correctly identify further opportunities to improve terminology consistency by translating "Store" as "ストア" instead of "店舗" in the PD scheduling best practices guide.

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