BR: Add a new doc about the batch create table

[en-jin19]

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• This PR is translated from:
• BR: Add a new doc about creating table in batch docs-cn#8659 ("TOC.md", "br/backup-and-restore-faq.md")
• Update the doc of BR batch create table docs-cn#8786 ("br/br-batch-create-table.md")
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• TomShawn

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[fengou1]

Suggested change

	The batch create table feature uses the concurrent batch table creation scheme. From v6.0.0, by default, BR creates tables in multiple batches, and each batch has 128 tables. Using this scheme, when BR creates one batch of tables, TiDB schema version only changes once. This scheme significantly increases the speed of table creation.
	The batch create table feature uses the concurrent batch table creation. From v6.0.0, by default, BR creates tables in multiple batches, and each batch has 128 tables. Using this new DDL job, when BR creates one batch of tables, TiDB schema version only changes once. This feature significantly improve the performance of table creation.

[en-jin19]

/verify

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All committers have signed the CLA.

[TomShawn]

LGTM

[en-jin19]

/remove-status LGT1
/status LGT2

[en-jin19]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 3b231b4

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8041.