docs 更新路由文档

[bibidu]

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/docs/routing.md

[codecov]

Codecov Report

Merging #4307 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4307   +/-   ##
=======================================
  Coverage   82.78%   82.78%           
=======================================
  Files         137      137           
  Lines        2701     2701           
  Branches      679      679           
=======================================
  Hits         2236     2236           
  Misses        456      456           
  Partials        9        9           

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