i18n(ja): unify the katakana term for "covering index"

[yahonda]

What is changed, added or deleted? (Required)

The ja docs mixed two renderings of English "covering index":

• カバーインデックス — 10 occurrences (6 files)
• カバリングインデックス — 2 occurrences (1 file)

This PR standardizes on カバリングインデックス, which is the faithful transliteration of "covering" and the standard term in Japanese database documentation. All 10 カバーインデックス occurrences (including the compound カバーインデックス最適化 / covering index optimization) are updated.

Heading text and link text are updated where they contain the term; the {#solution-use-covering-index} / {#sql-tuning-with-a-covering-index} anchors are kept unchanged, so existing links still resolve.

Files: develop/dev-guide-index-best-practice.md, develop/dev-guide-optimize-sql.md, develop/dev-guide-sql-development-specification.md, explain-indexes.md, sql-tuning-best-practice.md, tidb-cloud/tidb-cloud-sql-tuning-overview.md.

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)

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

N/A — terminology unification on the ja translation branch.

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