[Doc] add custom Icon directions

[qianzhaoy]

文档： Icon 组件文档增加如何添加自定义图标的方法

[codecov-io]

Codecov Report

Merging #754 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #754   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         104      104           
  Lines        2116     2116           
  Branches      319      319           
=======================================
  Hits         1922     1922           
  Misses         73       73           
  Partials      121      121

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[chenjiahan]

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