release-8.5: doc updates for v8.5.6

[hfxsd]

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• This PR is translated from:Feature/preview v8.5.6 docs-cn#21521
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  Revert "Revert "v8.5.6 dashboard: Add note about prepared statements … #22672)

[gemini-code-assist]

Code Review

This pull request updates TiDB documentation for v8.5.6 and v9.0.0, transitioning background task management to GA and introducing tidb_slow_log_rules and tidb_slow_log_max_per_sec for advanced slow query filtering. It also documents FOR UPDATE OF enhancements, the new tidb_max_dist_task_nodes variable, and updated sync-diff-inspector installation methods. Feedback focuses on style guide compliance by using the second person, improving readability through list formatting, and ensuring descriptions are concise and complete.

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[hfxsd]

Suggested change

	| [sync-diff-inspector](/sync-diff-inspector/sync-diff-inspector-overview.md) | For TiDB v8.5.6 and later: `tiflow-{version}-linux-{arch}.tar.gz` <br/>For versions before v8.5.6: `sync_diff_inspector` |
	| [sync-diff-inspector](/sync-diff-inspector/sync-diff-inspector-overview.md) | For TiDB v8.5.6 and later: `tiflow-{version}-linux-{arch}.tar.gz` <br/>For versions before v8.5.6: `sync_diff_inspector` |