Update dashboard FAQ

[shhdgit]

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)

Added a new FAQ for TiDB Dashboard

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

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

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

• Other reference link(s): tidb-dashboard#902

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

[TomShawn]

/label needs-cherry-pick-5.0
/label needs-cherry-pick-master
/translation done
/cc @breeswish
/assign @shhdgit

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• breeswish

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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Reviewer can indicate their review by writing /lgtm in a comment.
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[TomShawn]

/label requires-version-specific-changes
/hold

[TomShawn]

/hold

[shhdgit]

Updated, PTAL.

[TomShawn]

@shhdgit Please help confirm whether this PR content applies to v5.1. Thanks~

[shhdgit]

Yes, v5.1 can also be applied.

[breezewish]

@TomShawn PTAL, thanks!

[qiancai]

This PR is closed because the author created a new PR (#6559) in master to cover the same changes.