The warning is produced on `cssnano` call. See below:
> Without `from` option PostCSS could generate wrong source map and will not find Browserslist config. Set it to CSS file path or to `undefined` to prevent this warning.
Reproduction:
```shell
mkdir work
cd work
npx docusaurus-init
cd website
yarn install
yarn run build
```
Environments:
- Node: 12.6.0
- Yarn: 1.17.3
* chore: move to monorepo
* lint all js file
* simplify circleCI
* fix failing tests
* fix tests due to folder rename
* fix test since v1 website is renamed
