releases: add release notes for v8.4.0

[Oreoxmt]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

releases: add release notes for v8.4.0

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• 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)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions))

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

• This PR is translated from: releases: add release notes for v8.4.0 docs-cn#18556
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-chi-bot]

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[qiancai]

Suggested change

	To enhance the performance of vector search, you can create and use [vector search indexes](vector-search-index.md). Note that TiDB vector search indexes rely on TiFlash. Therefore, before using vector search indexes, make sure that TiFlash nodes are deployed in your TiDB cluster.
	To enhance the performance of vector search, you can create and use [vector search indexes](vector-search-index.md). Note that TiDB vector search indexes rely on TiFlash. Before using vector search indexes, make sure that TiFlash nodes are deployed in your TiDB cluster.

[qiancai]

Suggested change

In v8.4.0, TiDB introduces an optimized implementation of the hash join operator to improve its execution efficiency. Currently, this optimized implementation of hash join applies only to inner join and outer join operations and is disabled by default. To enable this optimized implementation, configure the [tidb_hash_join_version](/system-variables.md#tidb_hash_join_version-new-in-v840) system variable to `optimized`.

In v8.4.0, TiDB introduces an optimized version of the hash join operator to improve its execution efficiency. Currently, the optimized version of the hash join applies only to inner join and outer join operations and is disabled by default. To enable this optimized version, configure the [tidb_hash_join_version](/system-variables.md#tidb_hash_join_version-new-in-v840) system variable to `optimized`.

[lilin90]

/test pull-verify

[lilin90]

Refine descriptions

[wuhuizuo]

/retest

[ti-chi-bot]

@Oreoxmt: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	d16fab5	link	true	/test pull-verify

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