更新文档：使用 customize-cra 来实现按需加载 antd-mobile

[clarkzsd]

Fixes #3094

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

• Make sure that you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a PR to keep commit history clear.
• Add some descriptions and refer relative issues for you PR.

Extra checklist:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update TypeScript definition for the component.
• Add unit tests for the feature.

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This change is 

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@285ad8f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3132   +/-   ##
=========================================
  Coverage          ?   60.05%           
=========================================
  Files             ?       92           
  Lines             ?     2278           
  Branches          ?      654           
=========================================
  Hits              ?     1368           
  Misses            ?      901           
  Partials          ?        9

Flag	Coverage Δ
#web	60.05% <ø> (?)

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[doxiaodong]

ref: https://ant.design/docs/react/use-in-typescript#Customize-Theme
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