docs: add faqs

[xiaohuoni]

参考

• Close FAQ #1421, Close 兼容ie的问题 #5053
• https://v2.umijs.org/zh/guide/faq.html
• https://alitajs.com/faq/build

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

---

View rendered docs/docs/faq.zh-CN.md

[codecov]

Codecov Report

Merging #5044 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5044   +/-   ##
=======================================
  Coverage   82.84%   82.84%           
=======================================
  Files         153      153           
  Lines        3381     3381           
  Branches      888      888           
=======================================
  Hits         2801     2801           
  Misses        573      573           
  Partials        7        7           

---

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[xiaohuoni]

无情 ci，改个标点符号都要挂我！

[ycjcl868]

不需要这样，已经有一个 @umijs/plugin-helmet 插件了，直接

import { Helmet } from 'umi';

还能兼容 SSR，多好

[afc163]

这里需要改下。

[xiaohuoni]

这个插件使用的是 react-helmet ，在混合应用中，ios 端 web 容器内，使用 react-helmet 有失效的情况。

[afc163]

👍

[sorrycc]

👍