*: Add TiDB Dashboard session docs (#6711)

[ti-chi-bot]

This is an automated cherry-pick of #6711

Signed-off-by: Breezewish me@breeswish.org

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)

Add TiDB Dashboard docs about sessions.

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

• master (the latest development version)
• v5.1 (TiDB 5.1 versions) Version number needs change
• v5.0 (TiDB 5.0 versions) Version number needs change
• v4.0 (TiDB 4.0 versions) Version number needs change
• 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)?

• This PR is translated from:
• 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:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

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

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Commit hash: a0cfa73