Add pprof information to seek-help documents

[wxiaoguang]

As the title.

1. Teach users how to get stack trace when slow/hanging/deadlock problem occurs. It helps users:

   • High Idle CPU usage (fresh install) #15542
   • High cpu comsuming cause gitea very slow #15143
   • always running processes from 1.15.3 #17138
   • 504 on /admin after Gitea upgrade because of large action table #17504
   • and so on

2. Improve zh-cn and zh-tw documents.

[codecov-commenter]

Codecov Report

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The diff coverage is n/a.

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  Branches        ?        0           
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  Hits            ?    41111           
  Misses          ?    43022           
  Partials        ?     6563           

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