adjust the order of titles under Reference in TOC (#12108)

[ti-chi-bot]

This is an automated cherry-pick of #12108

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)

According to the user visit data of the official documentation website in 2022, tiflash, configuration files, and system variables are obviously more frequently searched than other pages. It is assumed that this is because these contents are not in an easy-catch position in the navigation tree. Therefore, this PR moves the storage engine part out, and moves system variables, CLI, and configuration files upward, hoping that users can more easily catch these pages.

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.

• master (the latest development version)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.4 (TiDB 6.4 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: adjust the order of titles under Reference in TOC docs-cn#12691
• 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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• shichun-0415

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[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: bc7a1a8