docs: add antd plugin config

[xiaohuoni]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

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View rendered docs/plugins/plugin-antd.md

[codecov]

Codecov Report

Merging #4938 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4938   +/-   ##
=======================================
  Coverage   82.98%   82.98%           
=======================================
  Files         152      152           
  Lines        3361     3361           
  Branches      896      878   -18     
=======================================
  Hits         2789     2789           
  Misses        565      565           
  Partials        7        7           

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[xiaohuoni]

umijs/plugins#283