update tidb.net and asktug links

[qiancai]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Replaced legacy community links across documentation files in this repository:
  • tidb.net/blog -> pingkai.cn/tidbcommunity/blog
  • search.asktug.com -> search.pingkai.cn
  • asktug.com -> pingkai.cn/tidbcommunity/forum
  • tidb.net -> pingkai.cn/tidbcommunity
• This update ensures all doc references consistently point to the new PingCAP community domains.

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• Delete files
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[qiancai]

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