chore: fix npm lost readme

[xrkffgg]

现状

现有根目录 README.md 是从 docs/index.md 软链接过来，为了同时在 websilte 显示和 github 中显示，又不维护多处。
但因为根目录下没有实际的 README.md，导致 https://www.npmjs.com/package/rc-field-form 没有显示介绍。

解决方案

为了不使得 README 维护两处，现移除 docs/index，根目录 README 使用实际内容。这样在 website 构建时目录为：

这个标题的显示 README 的问题看这里：umijs/dumi#408

为了暂时规避掉这个标题的问题，又在 umi 的 config 里强行用 js 改了

等 umijs/dumi#408 解决后，可移除这段 script

[vercel]

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🔍 Inspect: https://vercel.com/react-component/field-form/q26vt37sf
✅ Preview: https://field-form-git-test.react-component.now.sh

[xrkffgg]

@crazyair @PeachScript

[codecov]

Codecov Report

Merging #237 (effaf94) into master (846bb0b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files          13       13           
  Lines         954      954           
  Branches      222      222           
=======================================
  Hits          953      953           
  Misses          1        1           

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[crazyair]

LGTM