release-5.4: update tidb.net and asktug links

[qiancai]

What is changed, added or deleted? (Required)

Batch-updated docs links from tidb.net, asktug.com, search.asktug.com, learn.pingcap.cn/learner, and support.pingcap.cn to their new Pingkai destinations across the release-5.4 docs.

Kept the TiDB performance map link in grafana-tikv-dashboard.md unchanged as requested.

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

• master (the latest development version)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 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)

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

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[qiancai]

Close this PR because the v5.4 docs have been archived at https://docs-archive.pingcap.com/tidb/v5.4 and will no longer receive new updates.