add release notes 8.5.6 (#22578)

[ti-chi-bot]

This is an automated cherry-pick of #22578

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• v8.5 (TiDB 8.5 versions)
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• This PR is translated from: add release notes 8.5.6 docs-cn#21445
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• Delete files
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[gemini-code-assist]

Code Review

This pull request introduces the release notes for TiDB v8.5.6, updating the table of contents, release index, and timeline. The feedback identifies a documentation bug in the release timeline where the link for version 8.5.6 incorrectly points to the 8.5.5 release notes instead of the correct file.

[hfxsd]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch master: #22755.