update vector search docs (#18779)

[ti-chi-bot]

This is an automated cherry-pick of #18779

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What is changed, added or deleted? (Required)

This PR moves 15 vector search docs from the tidb-cloud folder to the vector-search folder to so they can be reused by TiDB self-managed docs.

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• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
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• v6.5 (TiDB 6.5 versions)
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Do your changes match any of the following descriptions?

• Delete files
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[qiancai]

/cc @EricZequan

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[EricZequan]

LGTM

[qiancai]

/approve

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