doc: batch processing overview and pipelined dml

[ekexium]

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What is changed, added or deleted? (Required)

1. A batch processing overview doc to disambiguate features
2. A standalone doc for Pipelined DML

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
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• v7.1 (TiDB 7.1 versions)
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• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

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[ti-chi-bot]

@ekexium: 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	54fb7a4	link	true	/test pull-verify

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