Add documents for `information_schema.tidb_index_usage` and `sys.schema_unused_index`.

[YangKeao]

What is changed, added or deleted? (Required)

Add the document for index usage related system tables and databases.

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).

• [x] master (the latest development version)
• [x] v8.0 (TiDB 8.0 versions)
• [ ] v7.6 (TiDB 7.6 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.4 (TiDB 7.4 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)
• [ ] v5.2 (TiDB 5.2 versions)
• [ ] v5.1 (TiDB 5.1 versions)
• [ ] v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: https://github.com/pingcap/docs/pull/16446
• 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[bot]]

@qw4990: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

REST LGTM

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Oreoxmt]

/cc @hfxsd

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-03-05 06:32:18.296308666 +0000 UTC m=+148765.318555049: :ballot_box_with_check: agreed by Oreoxmt.
• 2024-03-05 06:57:59.715702758 +0000 UTC m=+150306.737949146: :ballot_box_with_check: agreed by hfxsd.

[YangKeao]

/hold

[YangKeao]

/unhold

[ti-chi-bot[bot]]

@bb7133: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

LGTM

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Oreoxmt]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

The full list of commands accepted by this bot can be found here.

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• ~~OWNERS~~ [Oreoxmt]

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