update vector search docs

[qiancai]

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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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For details, see tips for choosing the affected versions.

• master (the latest development version)
• 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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• v7.1 (TiDB 7.1 versions)
• 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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[Oreoxmt]

/approve cancel

[ti-chi-bot]

PR needs rebase.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-03 02:39:01.388127072 +0000 UTC m=+325665.906179996: ☑️ agreed by Oreoxmt.
• 2024-10-21 08:29:53.288758545 +0000 UTC m=+253393.985549154: ☑️ agreed by hfxsd.

[lilin90]

/lgtm

[EricZequan]

The rest LGTM

[ti-chi-bot]

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[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.4: #19186.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #19189.